Improve src/ layout #672

Continued from #670 (comment)

It may be preferable to move pages in directories so that they follow the summary sidebar more. The downside is that then we are breaking external links to us, and we can't leave 301s from the old pages (we can try bodging it with client-side redirects, but... ick.)

Ironically, Codeberg Pages has support for those, and @Rangi42 checked and found out that GitHub Pages appears to be the odd one out here, against Netlify, Cloudflare Pages, and now Grebedoc/Codeberg Pages.