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.

models/face/README.md

@@ -29,7 +29,7 @@ directory as the pair, so a hand-placed pair does not pick up a package's eye mo
 directory it otherwise outranks.
 
     scrfd_500m_640.int8.onnx   0.8 MB   the same three, in the form the Hexagon NPU takes
-    scrfd_2.5g_640.int8.onnx   0.9 MB   (docs/inference.md §5) — opset 17, per-channel int8
+    scrfd_2.5g_640.int8.onnx   0.9 MB   (docs/dev/inference.md §5) — opset 17, per-channel int8
     scrfd_10g_640.int8.onnx    4.3 MB   weights, uint8 activations, calibrated on 96 photographs
 
 The int8 files are **derived** by `tools/quantise-models.sh` from the f32 ones beside them and
@@ -53,12 +53,12 @@ cannot parse any of the graphs while they are dynamic:
     ./tools/fix-face-model-shapes.sh 2d106det.onnx  models/face/2d106det_b1.onnx    --dim None=1
 
 `2d106det_b1.onnx` is from the same `buffalo_l.zip` as `det_10g.onnx` and under the same grant: the
-106-point landmark model whose lid contours the eye boxes are cut from (docs/faces.md §17.2).
+106-point landmark model whose lid contours the eye boxes are cut from (docs/dev/faces.md §17.2).
 sha256 as fetched `f001b856…a7109dbf`, as shipped `afc2984c…03368ef26`.
 
 The weights carry a non-commercial research-only grant. They are here because this is a private
 repository and self-installed builds; they come back out before anything is published, and the
-restriction binds whoever uses the app, not only the project. docs/faces.md §2.2a is the decision.
+restriction binds whoever uses the app, not only the project. docs/dev/faces.md §2.2a is the decision.
 
 ## The two classifiers are a different matter
 
@@ -96,4 +96,4 @@ Attribution, as ODC-By asks for the eye dataset:
 
 The variant choice was measured, not taken from the F1 column: on family snapshots the S variant
 of OCEC read more open eyes as open than M or L did, which overfit their own domain
-(docs/faces.md §17).
+(docs/dev/faces.md §17).