doc: shift doc to Mumbai

Context

This MR establishes the doc for the activation of Mumbai. For that, it follows meta-issue #2170 (which see for details).

Manually testing the MR

Check that the CI is green, that the doc compiles locally with no errors, then check that:

• the Lima section has been removed from the doc
• the active protocol is now Mumbai (follow any link containing "active", such as https://tezos.gitlab.io/active/rpc.html)
• navigating back and forth from shell pages to protocol pages (e.g. the RPC static references) works well.

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR