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.

tools/export-migan.sh

@@ -9,7 +9,7 @@
 #   ./tools/export-migan.sh    # -> models/inpaint/migan-512.onnx
 #
 # Since 2026-09-20 the file that ships is not this export but a fine-tune of
-# it on panorama-border voids (docs/panorama.md §14), made in the
+# it on panorama-border voids (docs/dev/panorama.md §14), made in the
 # `darkroom-infill` repository with `python -m infill.export`. This script
 # still yields the stock generator — the fine-tune's starting point, and the
 # model the page's "mirror depth" knob above zero was built around.