doc: shift doc to Nairobi

Context

This MR updates the documentation upon activation of protocol Nairobi. It is an instance of meta-issue #2170 (which see).

This mostly consists in dropping the doc pages for Mumbai, and switching the active protocol doc to Nairobi.

However, it also:

• refreshes the OS versions for which installation of Octez is tested daily
  • On Debian, Buster (v10) is dropped; Bullseye (v11) is now supported
  • On Ubuntu, Bionic (18.04) is dropped; Focal (20.04) and Jammy (22.04) are now supported
  • on Fedora, v36 is dropped; v37 and v38 are now supported
• patches some files unrelated to the update, in order to make the CI green, e.g.
  • fixes broken links
  • fixes some entries in the glossary

Manually testing the MR

Make the doc, locally and/or in the CI, and check that there are no errors.

• Contemplate the left menu: the section for Mumbai should be gone.
• All references to protocol pages or labels should go to Nairobi now (e.g., navigate from shell/cli-commands.rst to the protocol page and back).
• Search for "mumbai" in the left menu's search box: it should only find it in history pages (the changelog, pages under protocols/ and releases/, as an old test network, and in the history of protocol environments).

Verify labels with make -C docs xrefscheck:

• no error should be reported

Check the installation scripts in the doc, either:

• locally with make -C docs scriptsindoccheck, or
• in the CI (stage doc)

Checklist

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• Select as Assignee the next person who should take action on that MR