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/fill.rs

@@ -95,7 +95,7 @@ pub struct Params {
     /// and, beyond the band being filled, still unknown. That is what the
     /// shipped model was trained on (a fine-tune of MI-GAN on voids cut
     /// from photographs the way a cylindrical merge cuts them, see
-    /// `docs/panorama.md` §14); a ring would give it a fold to continue.
+    /// `docs/dev/panorama.md` §14); a ring would give it a fold to continue.
     ///
     /// Non-zero is the stock model's crutch: a plain reflection of a deep
     /// hole pulls in whatever is that far from the edge — a ridge, a peak —