Add docs last-updated metadata

[katie-z-geer]

Adds per-page freshness metadata to the docs, generated from the same mdBook source as the rest of the AI-readiness artifacts.

• Derives each page's last-updated date from Git history, with a committed docs/last-updated.json fallback for shallow CI/deploy checkouts.
• Adds <lastmod> to docs/sitemap.xml.
• Emits last-modified and article:modified_time meta tags in the built HTML.
• Annotates the generated llms.txt index and Markdown copies with last-updated dates.

Split out of #59577 so the freshness pipeline — and the shallow-checkout robustness it needs — can be reviewed on its own.

Stacked on

Based on docs/ai-readiness (#59577), not main. Set the PR base to docs/ai-readiness and merge that first, or retarget to main once it lands. The freshness code plugs into the ai_discovery module that #59577 introduces.

Validation

• cargo check -p docs_preprocessor
• cargo test -p docs_preprocessor (9 passed)
• ./script/clippy -p docs_preprocessor

Suggested .rules additions

N/A

Release Notes:

• N/A