shift doc to Oxford2

What

This MR adapts the doc for the activation of Oxford.

NB: This MR should be ready to be merged before the activation**, but not merged until activation,** currently estimated on Feb 9 at noon.

Why

Any links to or mentions of the active protocol must now point to Oxford, instead of Nairobi. The Nairobi pages are no more needed.

How

This MR follows the steps in meta-issue #2170 and groups the changes based on their kind, in several commits, for facilitating reviews. Basically, the changes drop directory docs/nairobi/, links Oxford as the active protocol (by adding non-suffixed labels in its pages), and adapts many other impacted files.

Manually testing the MR

Check that the CI is green.

Run the install tests in CI if not done. Note that installing the RC on Ubuntu Focal and Jammy fails, but this is already the case on master, and is due to the fact that renaming the teztnets website from ".xyz" to ".com" has not been back-ported to the RC packages.

Then navigate in the doc artifact and check that:

• the section on Nairobi in the left menu is gone
• any links containing active/ go to the Oxford pages (e.g. the first link in page INTRODUCTION/ Running Octez, on line To learn more about the protocol refer to this page. )
• in section CHANGES IN PROTOCOL VERSIONS, the release page of protocol Nairobi has moved to the subsection Older Protocols

Optionally, run some checks locally:

• make -C docs xrefscheck to check that labels are well-formed in the protocol-dependent pages; no warning should be raised
• make -C docs to build the doc locally; this should succeed

Checklist

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