Add CI check for release notes nav/index/date wiring

[ImTheKai]

Adds a read-only CI check (scripts/check_release_notes.py) that runs on any PR touching release notes, mkdocs.yml/mkdocs-base.yml, or variables.yml. It never edits or commits anything - it only validates and annotates the PR.

It catches the exact class of bug we just found and fixed manually for 17.11.1 and 18.6.1: a release notes page was added but never wired into the mkdocs nav, so it never showed up in the Release notes sidebar even though the page itself built fine.

Checks:

• every release-notes-vX.Y.Z.md page is referenced in the effective mkdocs nav (fails the check if missing)
• every such page is linked from the release notes index page (fails if missing)
• every page using the {{date.X}} macro has a matching variables.yml entry (fails if the key is missing entirely, since the macro would render blank)
• if that date is present but still looks like a placeholder (e.g. TBD), it's flagged as a warning only (not a failure), since the real date often isn't known yet at authoring time

Tested against the current state of all 5 branches (14/15/16/17/18, including 15's flat docs/release-notes-vX.md layout) - all currently pass clean. Also verified it correctly fails when the nav entry is missing (regression test against the exact bug we fixed) and correctly warns-without-failing on a TBD date.