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.

tools/bench/src/fixture.rs

@@ -1,6 +1,6 @@
 //! The synthetic 50k catalog, and the handful of real files it points at.
 //!
-//! `docs/requirements.md` §8 asks for "an automated benchmark suite against a
+//! `docs/dev/requirements.md` §8 asks for "an automated benchmark suite against a
 //! synthetic 50k catalog". The hard part of that sentence is *50k*: a real
 //! library of that size is several terabytes and cannot live in a repository,
 //! in a CI cache, or on a laptop that also has to compile the thing.