docs(wiki): flatten to root pages (Forgejo wikis are not nested)
Forgejo/Gitea wikis are flat — pages in real subdirectories 404 (only the page
title may contain a "/", stored as an escaped flat file). The folder layout broke
every moved page on the live wiki. Move all pages back to the repo root and make
links bare page names again. Hierarchy is now expressed via _Sidebar.md headings
(slashes removed).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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