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/Cargo.toml

@@ -47,10 +47,10 @@ dr-lens.workspace = true
 dr-pipeline.workspace = true
 dr-catalog.workspace = true
 # The face pipeline, with the ONNX runtime: this is the layer that actually
-# runs the models over the library (docs/faces.md).
+# runs the models over the library (docs/dev/faces.md).
 dr-face = { workspace = true, features = ["inference"] }
 # The engine behind both. `native` here means the *app* may look for a
-# runtime file; the build stays C-free either way (docs/inference.md §3).
+# runtime file; the build stays C-free either way (docs/dev/inference.md §3).
 dr-inference-engine = { workspace = true, features = ["native"] }
 dr-thumbs.workspace = true
 # For the drag ghost only: the composite under the cursor has to reach the