…; rebuild guidance page

- redirects.njk was inheriting the site layout instead of rendering as a
  bare redirect stub (missing layout: false), so /about.html rendered
  55KB of duplicate page chrome with a generic title/no description
  instead of actually redirecting - this was the real source of the
  reported "OG title missing" / "OG description missing" on /about
- Added a description to the homepage (index.njk), fixing both the
  missing page description and OG description there
- Added id="main-content" to 5 layouts (search, site-index, post, cards,
  home) whose <main> didn't match the skip-link's #main-content target
- dropdown-menu-end on the Share/Download dropdowns, plus icon color
  rules synced from components (fa-expand, fa-terminal, fa-rocket)
- Rebuilt guidance.md from guidance.njk: a plain intro line + linked
  list of the guidance sources, dropping the old guidance-to-standards
  table