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.

core/dr-face/src/detect.rs

@@ -1,4 +1,4 @@
-//! SCRFD face detection (docs/faces.md §4).
+//! SCRFD face detection (docs/dev/faces.md §4).
 //!
 //! One forward pass produces a box, a confidence and **five landmarks** per
 //! face — the landmarks being the reason for this detector rather than a
@@ -138,7 +138,7 @@ impl Detection {
 pub struct Detector {
     session: Model,
     /// f32 or int8 — the int8 form finds a different set of faces and is a
-    /// different detector in `model_id` (docs/inference.md §7).
+    /// different detector in `model_id` (docs/dev/inference.md §7).
     form: Form,
     /// Feature-map count: 3 for strides {8,16,32}, 4 for {8,16,32,64}.
     ///
@@ -346,7 +346,7 @@ fn iou(a: &(f32, f32, f32, f32), b: &(f32, f32, f32, f32)) -> f32 {
 /// How the image is fitted into the graph's fixed square input.
 ///
 /// The forward and inverse mappings live in one struct on purpose:
-/// docs/faces.md §4.1 notes that what matters is not *where* the padding goes
+/// docs/dev/faces.md §4.1 notes that what matters is not *where* the padding goes
 /// but that the two agree. A mismatch offsets every box and landmark by the
 /// padding, producing detections that look plausible and embeddings that
 /// quietly cluster badly three stages later.
@@ -372,7 +372,7 @@ impl Letterbox {
     ///
     /// `(x·255 − 127.5) / 128` — note `/128`, not `/127.5`. The reference
     /// implementation this is ported from uses `/128` for both models, and
-    /// every measured number in docs/faces.md §1 came from it.
+    /// every measured number in docs/dev/faces.md §1 came from it.
     ///
     /// Padding is grey, matching the reference's `114`: the value the network
     /// reads least as an edge, where black would draw a hard border across the