docs: add missing 2.12 upgrade page to nav menu

[fletch3555]

This PR targets 2.12 docs directly as it has already been resolved in release-2.13 and master.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

[blakepettersson]

LGTM, once you signoff your commit

[fletch3555]

LGTM, once you signoff your commit

Is it not?

The DCO check is successful which (as far as I was aware) indicates the commits are properly signed-off. I do see several failed E2E tests, but not sure why that is since I didn't touch any code.

I'm new to contributing to this project, so a little bit of guidance/hand-holding would be very much appreciated. Thanks!

[blakepettersson]

@fletch3555 nm, I could've sworn that was an issue - in any case LGTM.