[paulbalandan]

Description
Part 3 of the user guide cutover in #10683, after #10714 and #10715.

The release scripts now read and write the Markdown guide in docs/:

• admin/next-changelog-minor.md, next-changelog-patch.md and next-upgrading-guide.md replace the RST templates.
• create-new-changelog.php creates docs/changelogs/v{version}.md and the upgrade guide stub, links both from index.md and upgrading.md, and adds them to docs/.nav.yml.
• update-upgrade-guide.php and generate-changelog.php use the new paths and the Markdown section headings.
• prepare-release.php and .github/scripts/validate-version update and check the ## Version X.Y heading of docs/changelogs/index.md.
• The three AutoReview tests and admin/RELEASE.md follow.

The |version| substitution in the RST changelog index was resolved by Sphinx from conf.py. The converted page has the version as plain heading text, so the release script now edits that heading. This departs from item 7 of #10683, which assumed a mkdocs.yml key.

The old RST templates and the epub and Sphinx steps in RELEASE.md stay until the switch PR.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide