Fix redirect/homepage meta tags, skip-link targets, dropdown overflow…

[lukefretwell]

…; 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 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