[TASK] Add RVFI specification to Reference Guide

Task Description

Generate CVE2-specific documentation for the RISC-V Formal Interface. This should be modeled after the similar chapter in the CV32E40X User Manual (link).

This was originally a sub-task of #185 (closed).

Description of Done

The new sub-chapter of the CVE2 Reference Guide has been merged into the repo and is rendering correcting on RTDs.

Associated PRs

No response