[paulbalandan]

Description
Part 2 of the user guide migration agreed in #10683. It builds on #10714, so it needs that PR merged first. The Sphinx build and the deployment are unchanged, and nothing publishes the Markdown yet.

What this adds

• mkdocs.yml and the Material theme in docs/overrides/, with docs/requirements.txt for the Python packages. The theme and the requirements file live in docs/ so that only mkdocs.yml is added at the repository root. exclude_docs keeps them out of the built site.
• admin/llms.py writes llms.txt, llms-full.txt and a Markdown copy of each page beside its HTML (item 8 of docs: decisions needed to migrate the user guide to MkDocs #10683). It reads only the nav and the Markdown, so it does not depend on MkDocs.
• admin/anchors.py adds the anchor ids that the Sphinx guide used (RST labels and PHP object ids) to the built HTML, so that old links with a #fragment still scroll to their section. The ids are listed in docs/.anchors.json, which keeps the Markdown clean.
• admin/update-docs-nav.php keeps docs/.nav.yml up to date. It adds a page that is missing from the nav to the section whose first page links to it, in the order of that list. --check changes nothing and exits with 1 when a page is missing or a nav entry has no file. It has a test in tests/system/AutoReview/UpdateDocsNavTest.php.
• A markdown_check job in test-userguide.yml that runs the nav check, the strict MkDocs build, llms.py and anchors.py. The job fails when a page is missing from docs/.nav.yml. Running php admin/update-docs-nav.php fixes it.
• .gitattributes excludes docs/ and mkdocs.yml from the Composer archive, .editorconfig keeps the two trailing spaces that the converted pages use as line breaks, and .gitignore covers site/, venv/ and __pycache__/.

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