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.

ui/dr-ui/src/repairs.rs

@@ -322,7 +322,7 @@ pub fn registry(
         // detector found on a proxy as found. The re-index converges on
         // provenance: every box, landmark, crop and vector from the current
         // detector over the native render, because those are what every
-        // later per-face pass reads (docs/faces.md §17.4a).
+        // later per-face pass reads (docs/dev/faces.md §17.4a).
         out.push(Repair {
             name: "face-detection",
             label: "images to detect faces in with the chosen detector",