clean FPU doc

Created by: davideschiavone

This PR aims to solve:

#255 (closed)

#253 (closed)

and it makes updates to the FPU part of the documentation.

In particular,

1. It updates the part related to latency and repository of the external FPU
2. It removes the redundat part of the instructions implemented as they are specified in the RISC-V standard (RV32F)