Add social cards and a favicon #135

Sharing a docs link anywhere (Slack, a tweet, a PR comment) produces a bare URL with no preview. mkdocs-material can generate an Open Graph card per page from the page title and description.

What to do:

1. Add the social plugin to mkdocs.yml.
2. Switch docs-requirements.txt to mkdocs-material[imaging].
3. Install the system dependencies in .github/workflows/docs.yml. This is the part that will bite you: the imaging extra needs cairo and pango, so the workflow needs an apt-get install libcairo2-dev libfreetype6-dev libffi-dev libjpeg-dev libpng-dev libz-dev step before pip install. Without it the build fails with a cairo import error.
4. Add a favicon while you are in there.

Acceptance:

• mkdocs build --strict produces site/assets/images/social/ with one image per page.
• The docs workflow passes on a PR, which is where the system-dependency step gets proven.
• Pasting a docs URL into a preview renderer shows a title card.

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