Put the developer docs under docs/dev and index the folder for users first

docs/ had 26 developer documents flat beside the manual, and the two
audiences are very differently sized: most readers want the manual and
the gesture reference, a few want the register, the designs and the
measurements. The manual and gestures.md stay at the top; everything for
someone changing the code moves to docs/dev/, and the two documents that
name their own successors — the v0.1 milestone and the UI-refinement plan
— go to docs/dev/archive/ rather than being deleted, since both are still
cited. docs/README.md is the index, users first.

Every reference follows: code comments, Cargo manifests, the workflows,
the pre-commit hook, the bench and traceability tools (which locate the
repo root by docs/dev/requirements.md now), packaging, the Docker READMEs,
CLAUDE.md, CONTRIBUTING.md and the README. The matrix links one level
deeper and is regenerated. Links out of the moved documents into the tree
gain a level; a link checker over every Markdown file finds none broken.

packaging/flatpak/paris.tourolle.darkroom.yml

@@ -8,7 +8,7 @@
 #     flatpak-builder --user --install --force-clean \
 #         build/flatpak packaging/flatpak/paris.tourolle.darkroom.yml
 #
-# Read docs/distribution.md before changing any permission below. Every line in
+# Read docs/dev/distribution.md before changing any permission below. Every line in
 # `finish-args` is a hole in the sandbox, and the one that is conspicuously
 # absent — `--filesystem=` — is absent on purpose and is explained there.
 id: paris.tourolle.darkroom
@@ -99,7 +99,7 @@ finish-args:
   # honest error.
   #
   # `--filesystem=host` would make that work today and is exactly what the
-  # requirement forbids, so it is not here. docs/distribution.md §4 records what
+  # requirement forbids, so it is not here. docs/dev/distribution.md §4 records what
   # closes the gap and how to run a Flatpak build in the meantime.
 
 modules: