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.

docker/windows/README.md

@@ -1,7 +1,7 @@
 # Windows cross-build environment
 
 Reproducible container for building the Windows executable and its installer from Linux. The
-specification is [docs/windows.md](../../docs/windows.md); this directory is what it turned into,
+specification is [docs/dev/windows.md](../../docs/dev/windows.md); this directory is what it turned into,
 and every departure from the spec's first draft is recorded in the Dockerfile's comments.
 
 ## Use
@@ -16,7 +16,7 @@ and every departure from the spec's first draft is recorded in the Dockerfile's
 # Build the installer from that binary
 ./docker/windows/build.sh docker/windows/package.sh
 
-# Smoke-test under Wine (docs/windows.md §6)
+# Smoke-test under Wine (docs/dev/windows.md §6)
 ./docker/windows/build.sh wine target-windows/x86_64-pc-windows-gnu/release/darkroom-desktop.exe --version
 ./docker/windows/build.sh wine target-windows/installer/DarkRoom-0.12.0-x86_64-setup.exe /S