Backport MicroCloud 3 release notes to MicroCloud 2 docs (v2-edge)

[elijahgreenstein]

This PR:

• backports the release notes for MicroCloud 3.1 and MicroCloud 3.2 to the documentation for MicroCloud 2 (from PRs Doc: Release notes 3.1 #1274, Doc: 2.1.3 release notes #1343, and Doc: 3.2 release notes #1345), and
• updates the MicroCloud 2 release notes TOC to use the pages, rather than links out to MicroCloud 3 (latest) docs.

Motivation: if a user is viewing the default version of the docs and navigates to the 3.1 or 3.2 release notes, then the user jumps to the latest version. If the user then uses the version selector flyout menu to navigate back to default, the user lands on a 404 page.

We could, alternatively, put in place a redirect for the /reference/release-notes/release-notes-3.{1|2}/ path back to latest, but if that additional step of adding a redirect is required anyway, it seems simpler to backport the MicroCloud 3 release notes. This also ensures that users do not unintentionally switch from the default to latest docs versions.

[elijahgreenstein]

@roosterfish -- another option is to simply limit the MicroCloud 2 docs release notes to MicroCloud 2 and earlier. This is the approach in Juju and Snapcraft for example:

• Snapcraft 8 docs only provide release notes up through 8: https://ubuntu.com/docs/snapcraft/8/release-notes/
• Snapcraft 9 docs provide release notes up through 9: https://ubuntu.com/docs/snapcraft/9/release-notes/
• Juju 2.9 provides release notes for 2.9 and earlier: https://canonical.com/juju/docs/juju-cli/2.9/
• Juju 3.6 provides release notes for 3.6, and < 2.9 (does not link to 2.9 itself): https://canonical.com/juju/docs/juju-cli/3.6/releasenotes/

[elijahgreenstein]

RTD build is failing, since the release notes include links to MicroCloud 3 content that is unavailable in MicroCloud 2. Possibly further supports the approach of limiting release notes to past versions, rather than adding future versions. Or adding redirects to latest.