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-pano/src/xfeat.rs

@@ -4,7 +4,7 @@
 //! Apache-2.0 weights (`models/LICENCE.md`), exported at a fixed shape by
 //! `tools/export-xfeat.sh` and loaded through the same `dr-inference-engine`
 //! `dr-segment` and `dr-face` use, so this adds no runtime and no C to the
-//! tree; what runs it is the device's business (docs/inference.md). ~300 ms
+//! tree; what runs it is the device's business (docs/dev/inference.md). ~300 ms
 //! per frame on tract on the reference desktop, ~400 ms on the tablet
 //! (S15.2, S15.4).
 
@@ -37,7 +37,7 @@ pub struct XFeat {
 }
 
 /// The bytes of both exports compiled into the binary, 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]; 2] {
     [EMBEDDED_LANDSCAPE, EMBEDDED_PORTRAIT]