Add social cards, asset self-hosting and page dates to the docs

[xhan97]

Adds three Material for MkDocs plugins. Two are built into the theme; the third is one dependency.

social

Renders a preview card per page, so a link to the docs unfurls with the page title and the project logo instead of a bare URL. 26 cards are generated.

It needs Material's imaging extra (cairosvg, pillow). Declaring mkdocs-material[imaging] also fixes something incidental: the theme was previously only reached transitively through mkdocs-jupyter, even though it is a direct requirement of this site.

Card URLs are absolute and built from site_url, which mike rewrites to include the version prefix during a versioned deploy, so they resolve under /dev/ and /0.3.0/ alike.

privacy

Downloads the Google Fonts the theme loads (Open Sans, Ubuntu Mono) and serves them from this site, removing the third-party request every visitor currently makes for them. Verified in the built output: zero remaining requests to fonts.googleapis.com or fonts.gstatic.com.

Four things are deliberately left external:

Asset	Why
Google Analytics	Only works when loaded from Google. Vendoring it breaks the tag and changes nothing about the tracking, so the consent banner stays.
README badges (shields.io, pepy, codecov, repostatus, the Actions badge)	Rendered on request by their providers. A local copy would freeze download counts, coverage and build status at build time.
unpkg.com/mermaid@11	The theme bundle names this URL so it can lazy-load Mermaid. No page here defines a diagram, and the file is 3.4 MB — which mike would keep for every version it has ever deployed.
kit.fontawesome.com	A kit is a loader, not the icons. Its config points at ka-f.fontawesome.com, so a local copy still reaches Font Awesome and only adds a way for the kit to break.

Excluding Mermaid and the kit takes the vendored assets from 4.7 MB to 1.3 MB per deployed version.

git-revision-date-localized

Shows when a page last changed, so a reader can judge whether what they are reading is current. Both docs workflows already check out with fetch-depth: 0, so the history it needs is available. Undateable pages fall back to the build date rather than failing the strict build.

CI

Cards and downloaded assets are written to .cache, now restored in both docs.yml and deploy-gh-pages.yml. A cold build fetches around thirty font files and mkdocs runs with --strict, so without the cache a single failed download would fail the build.

Verification

mkdocs build --strict passes locally. In the output: 26 social cards, og:image pointing at the right card, fonts served from assets/external/, the gtag script still external, and the "Last update" fact rendering on each page. llms.txt and the 26 per-page .md exports from #69/#70 are unaffected.

The one thing not verified locally is that cairosvg imports on the runner — it needs libcairo from the system. It is present on ubuntu-latest images, and the docs job on this PR proves it either way before anything merges.

🤖 Generated with Claude Code