SEO: per-page meta descriptions, social cards plugin, and slimmer crawl surface for the docs site

[hexsecs]

Problem

• mkdocs-material reuses the global site_description ("CLI-first CAN security research toolkit documentation") as the meta description for every page unless the page sets description: front matter. Right now ~100 pages ship identical meta descriptions.
• The cookbook/tutorial titles ("Decode SPN 110", "Fuzz Tesla DI_torque2 vehicle speed", "Verify a PCAN interface in 30 seconds") match real search queries almost verbatim — these are the project's best long-tail search assets and deserve individual descriptions.
• ~70 nav pages are internal design/test specs, which dilutes what search engines see as the site's focus.
• No per-page OG images for docs pages.

Suggested fix

• Add description: front matter to the high-value pages: getting_started.md, feature-matrix.md, event-schema.md, security-use-cases.md, every cookbook/*.md, every tutorials/*.md (~30 pages)
• Enable the built-in social plugin in mkdocs.yml (free in mkdocs-material; needs the cairo image deps in the docs CI job) to auto-generate OG card images per docs page
• Consider excluding design/ and tests/ spec pages from the sitemap/index (e.g. mkdocs-exclude-search or a robots meta via template override), or grouping them behind a single "Internals" entry — keep them public, just de-emphasized

Acceptance criteria

High-value docs pages have unique meta descriptions; sharing a docs URL produces a per-page social card; the sitemap emphasizes user-facing pages.