The mdBook migration split the manual into one HTML page per section,
but same-page fragment links (e.g. [Standalone setup](#sec-flakes-standalone))
were left pointing at anchors that now live on other pages, so they
silently do nothing when clicked.
Rewrite all cross-page fragment links in docs/manual and
docs/release-notes to page-qualified relative links. Option anchor
links (#opt-*, #nixos-opt-*, #nix-darwin-opt-*) are untouched since
convert-markup.py rewrites those at build time.
Also give the previously empty-label [](#sec-tests) link in the 25.05
release notes a "Tests" label, since the mdBook renderer does not
auto-fill labels from the target heading.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
