What is claimed is:1. An information handling system, comprising:a motherboard comprising a mechanical post extending from a surface of the motherboard; anda riser card assembly comprising:a circuit board;a cable configured to communicatively couple the circuit board to the motherboard;a connector configured to receive an expansion card to communicatively couple the expansion card to the motherboard; anda detent assembly configured to mechanically couple the riser card assembly to the motherboard, the detent assembly comprising:a pin mechanically coupled to a surface of the riser card assembly and extending from the surface of the riser card assembly and configured to mechanically engage with a first feature of the mechanical post in order to align the riser card assembly to the motherboard; anda detent mechanically coupled to the pin and having an annular shape, the detent configured to mechanically engage with a second feature of the mechanical post in order to mechanically retain the riser card assembly with the motherboard.2. The information handling system of claim 1, wherein a location of the mechanical post is defined by a standard.3. The information handling system of claim 1, wherein:the first feature comprises a hole formed in the mechanical post; andthe pin includes an alignment feature configured to engage with the hole in the mechanical post in order to align the riser card assembly to the motherboard.4. The information handling system of claim 1, wherein the second feature comprises a shelf extending from a body of the mechanical post.5. The information handling system of claim 4, wherein the detent comprises:a base having a generally circular shape with a surface generally in parallel with a surface of the riser card assembly;a plurality of retention arms extending away from the base in a direction perpendicular to the surface of the base and each retention arm of the plurality of retention arms having an annular shape that is generally concentric with the generally circular shape defined by the base; anda plurality of engagement features, each engagement feature extending from a corresponding extension arm in a direction generally parallel to the surface of the base and configured to engage with the shelf to mechanically retain the riser card assembly with the motherboard when the detent assembly is engaged with the mechanical post.6. The information handling system of claim 5, wherein:the plurality of engagement features are biased in a first position relative to the base due to a spring force between the engagement features of the body; andthe plurality of engagement features are configured to engage with the shelf to overcome the bias in the first position such that the engagement features move from a first side of the shelf to a second side of the shelf when the detent assembly is engaged with the mechanical post in order to mechanically retain the plurality of engagement features with the second side of the shelf to mechanically retain the riser card assembly with the motherboard.7. A riser card assembly comprising:a circuit board;a cable configured to communicatively couple the circuit board to a motherboard;a connector configured to receive an expansion card to communicatively couple the expansion card to the motherboard; anda detent assembly configured to mechanically couple the riser card assembly to the motherboard, the detent assembly comprising:a pin mechanically coupled to a surface of the riser card assembly and extending from the surface of the riser card assembly and configured to mechanically engage with a first feature of a mechanical post in order to align the riser card assembly to the motherboard; anda detent mechanically coupled to the pin and having an annular shape, the detent configured to mechanically engage with a second feature of the mechanical post in order to mechanically retain the riser card assembly with the motherboard.8. The riser card assembly of claim 7, wherein a location of the mechanical post is defined by a standard.9. The riser card assembly of claim 7, wherein:the first feature comprises a hole formed in the mechanical post; andthe pin includes an alignment feature configured to engage with the hole in the mechanical post in order to align the riser card assembly to the motherboard.10. The riser card assembly of claim 7, wherein the second feature comprises a shelf extending from a body of the mechanical post.11. The riser card assembly of claim 10, wherein the detent comprises:a base having a generally circular shape with a surface generally in parallel with a surface of the riser card assembly;a plurality of retention arms extending away from the base in a direction perpendicular to the surface of the base and each retention arm of the plurality of retention arms having an annular shape that is generally concentric with the generally circular shape defined by the base; anda plurality of engagement features, each engagement feature extending from a corresponding extension arm in a direction generally parallel to the surface of the base and configured to engage with the shelf to mechanically retain the riser card assembly with the motherboard when the detent assembly is engaged with the mechanical post.12. The riser card assembly of claim 11, wherein:the plurality of engagement features are biased in a first position relative to the base due to a spring force between the engagement features of the body; andthe plurality of engagement features are configured to engage with the shelf to overcome the bias in the first position such that the engagement features move from a first side of the shelf to a second side of the shelf when the detent assembly is engaged with the mechanical post in order to mechanically retain the plurality of engagement features with the second side of the shelf to mechanically retain the riser card assembly with the motherboard.13. A detent assembly configured to mechanically couple a first mechanical member to a second mechanical member, the detent assembly comprising:a pin mechanically coupled to a surface of the first mechanical member and extending from the surface of the first mechanical member and configured to mechanically engage with a first feature of a mechanical post of the second mechanical member in order to align the first mechanical member to the second mechanical member; anda detent mechanically coupled to the pin and having an annular shape, the detent configured to mechanically engage with a second feature of the mechanical post in order to mechanically retain the first mechanical member with the second mechanical member.14. The detent assembly of claim 13, wherein:the first feature comprises a hole formed in the mechanical post; andthe pin includes an alignment feature configured to engage with the hole in the mechanical post in order to align the first mechanical member to the second mechanical member.15. The detent assembly of claim 13, wherein the second feature comprises a shelf extending from a body of the mechanical post.16. The detent assembly of claim 15, wherein the detent comprises:a base having a generally circular shape with a surface generally in parallel with a surface of the first mechanical member;a plurality of retention arms extending away from the base in a direction perpendicular to the surface of the base and each retention arm of the plurality of retention arms having an annular shape that is generally concentric with the generally circular shape defined by the base; anda plurality of engagement features, each engagement feature extending from a corresponding extension arm in a direction generally parallel to the surface of the base and configured to engage with the shelf to mechanically retain the first mechanical member with the second mechanical member when the detent assembly is engaged with the mechanical post.17. The detent assembly of claim 16, wherein:the plurality of engagement features are biased in a first position relative to the base due to a spring force between the engagement features of the body; andthe plurality of engagement features are configured to engage with the shelf to overcome the bias in the first position such that the engagement features move from a first side of the shelf to a second side of the shelf when the detent assembly is engaged with the mechanical post in order to mechanically retain the plurality of engagement features with the second side of the shelf to mechanically retain the first mechanical member with the second mechanical member.