Add crypto/x509 to FIPS User Guide

[qmuntal]

crypto/x509 was not initially added to the FIPS User Guide on purpose, as it has not been modified as part of the FIPS 140-2 support. On the other hand, it does use many crypto primitives that are affected by the FIPS support.

As this is such a foundational package, I makes sense to document it in the FIPS User Guide, at least the things related to the encryption and signing APIs.

Context: #692 (comment)