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-segment/src/semantic.rs

@@ -1,4 +1,4 @@
-//! Semantic segmentation — arm B (S15, docs/segmentation.md §4).
+//! Semantic segmentation — arm B (S15, docs/dev/segmentation.md §4).
 //!
 //! Runs a YOLO instance-segmentation graph over a proxy-resolution image and
 //! returns the instances it found: a class, a score, a box, and a soft mask
@@ -209,7 +209,7 @@ const EMBEDDED_MODEL: &[u8] = include_bytes!("../../../models/segment/yolo26n-se
 const EMBEDDED_CLASSES: &str = include_str!("../../../models/segment/yolo26n-seg.classes.json");
 
 /// The bytes of the model that ships with this crate, for whoever compiles
-/// engines ahead of the first request (docs/inference.md §6).
+/// engines ahead of the first request (docs/dev/inference.md §6).
 #[cfg(feature = "embedded-model")]
 pub fn embedded_model_bytes() -> &'static [u8] {
     EMBEDDED_MODEL
@@ -237,7 +237,7 @@ impl SemanticModel {
 
     pub fn from_bytes(bytes: &[u8], classes: Vec<Arc<str>>) -> Result<Self, SegmentError> {
         // The f32 graph on whatever the device's backend is. An int8 form
-        // for the Hexagon waits on docs/inference.md §10 M7 — the mask
+        // for the Hexagon waits on docs/dev/inference.md §10 M7 — the mask
         // boundary has to be measured before it moves.
         let session = dr_inference_engine::open(
             dr_inference_engine::Role::Segmenter,