Improve `src/` layout

*Continued from https://github.com/gbdev/pandocs/issues/670#issuecomment-5654058996*

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](https://docs.codeberg.org/codeberg-pages/advanced-usage/#redirects), 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.

Start by reviewing the current src/ directory layout against the summary sidebar structure. Document which page moves would be needed, then investigate how GitHub Pages handles the resulting external links and whether a redirect approach is viable; done means an agreed layout and redirect strategy.

Written by the indexing model from the issue text.