Backfill per-page SEO/OG front matter across all modules (#88)

Extends the pilot convention (PR #107) to the rest of the textbook: adds
`description:` to every lesson/index page and `image:` per the established rule
(static PNG/JPG/GIF hero → that asset; YouTube hero → hqdefault thumbnail; MP4
hero → auto-generated poster; otherwise omit → site card).

98 pages across electronics, arduino (remaining), sensors, advancedio,
communication, esp32, signals, cpx, microcontrollers, resources, tools, and the
site landing page. 16 new OG posters generated from MP4 heroes via
scripts/generate_og_posters.py.

Excluded: deprecated arduino/potentiometers-old.md and the root contributor docs
(website-dev/install, teaching-notes, website-content-ideas).

Verified: full production build succeeds; every local image: path resolves to a
file on disk; og:image/og:description render correctly for static, YouTube, and
poster hero types.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jonfroehlich