Updates to “base” CV32E20 User Manual

[DBees]

This task is complete when:

• all IBEX-specific content is either removed or deprecated.
• all CSRs have been updated.
• the RVFI implementation is fully documented.
• Mike stops complaining about the state of the User Manual! 😆

[MikeOpenHWGroup]

Current status:

• all IBEX-specific content is either removed or deprecated.
  See PR Docs update #293

• all CSRs have been updated.
  See PR Docs update #293

• the RVFI implementation is fully documented.
  Under review

• Mike stops complaining about the state of the User Manual! 😆
  Never going to happen! 😆