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-inference-engine/src/lib.rs

@@ -1,5 +1,5 @@
 //! Which runtime, which provider and which model form — decided once per
-//! device, and the only crate that knows the answer (docs/inference.md).
+//! device, and the only crate that knows the answer (docs/dev/inference.md).
 //!
 //! Consumers ask for a session by [`Role`] and get `ort`'s `Session` back;
 //! what built it — tract on one core, ONNX Runtime's CPU pool, a TensorRT
@@ -35,13 +35,13 @@ pub enum Role {
     Embedder,
     Segmenter,
     Scene,
-    /// The dense landmark model behind the eye reading (docs/faces.md §7c).
+    /// The dense landmark model behind the eye reading (docs/dev/faces.md §7c).
     Landmarks,
     /// The eye-state and sunglasses classifiers, a few hundred kilobytes.
     EyeClassifier,
-    /// XFeat, the panorama keypoint detector (docs/panorama.md).
+    /// XFeat, the panorama keypoint detector (docs/dev/panorama.md).
     Keypoints,
-    /// MI-GAN, the panorama border filler (docs/panorama.md §12). Plain
+    /// MI-GAN, the panorama border filler (docs/dev/panorama.md §12). Plain
     /// convolutions, so any rung serves it; fp16 on TensorRT and int8 on
     /// the Hexagon are the point of it.
     Inpainter,
@@ -516,7 +516,7 @@ mod tests {
 
     /// The smallest shipped graph, if this checkout has the weights; a test
     /// suite that needs a research-licensed download is one that does not
-    /// run in CI (docs/faces.md §3), so absence is a skip.
+    /// run in CI (docs/dev/faces.md §3), so absence is a skip.
     fn probe_bytes() -> Option<Vec<u8>> {
         let path = concat!(
             env!("CARGO_MANIFEST_DIR"),