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.

docker/android/README.md

@@ -48,7 +48,7 @@ pointers there; the build detects that and stops rather than shipping them.
 
 This exists because Android offers no other route to a model: the directory the app reads from is
 inside app-private storage, `run-as` needs a debuggable build, and the app has no picker and no
-fetch. docs/faces.md §2.2a is the decision and its limits — these files come back out before anything
+fetch. docs/dev/faces.md §2.2a is the decision and its limits — these files come back out before anything
 is published.
 
 ## Java in the APK