Add llms.txt generation

[thomasbergernz]

Summary

• Add mkdocs-llmstxt plugin so mkdocs build emits public/llms.txt — a machine-readable index for LLMs per the llms.txt spec.
• Sections mirror the top-level nav order in docs/.pages.yml, plus a Reference section for GLOSSARY.md/index.md. Root contributor/meta pages (CONTRIBUTING.md, FORMAT.md, NEWPAGE.md, MACROS.md, tags.md, mermaid-test.md, updates.md) and the stale, content-less General//Scientific_Computing/ dirs are excluded.
• llms.txt only — no llms-full.txt, to avoid a multi-MB addition to the Pages deploy artifact across 306 pages.
• requirements.in/requirements.txt updated (pip-compile) to pin mkdocs-llmstxt==0.5.0 and its deps (beautifulsoup4, markdownify, mdformat, mdformat-tables) — no version conflicts with the existing mkdocs < 2.0 pin.

Test plan

• mkdocs build --clean succeeds locally, public/llms.txt has all 11 sections with working links
• ./checks/run_test_build.py from a clean state: exit 0, no llmstxt-related warnings (pre-existing nav/redirect warnings unchanged)
• checks/run_aria_check.py passes — only globs *.html, unaffected by the new sibling .md files the plugin writes into public/
• CI (checks.yml) green on this PR
• PR preview (demo_deploy.yml): fetch <preview-url>/llms.txt and spot-check a couple linked pages

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