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.

.gitea/workflows/build-and-test.yml

@@ -322,7 +322,7 @@ jobs:
         env:
           CARGO_TARGET_DIR: target-android
           # Absent secrets mean a debug signature, which is what a fork or a
-          # branch build should get. Set all three (see docs/android-signing.md)
+          # branch build should get. Set all three (see docs/dev/android-signing.md)
           # and the same job produces a release-signed APK instead.
           ANDROID_KEYSTORE_BASE64: ${{ secrets.ANDROID_KEYSTORE_BASE64 }}
           KEYSTORE_PASS: ${{ secrets.ANDROID_KEYSTORE_PASSWORD }}
@@ -370,7 +370,7 @@ jobs:
 
   # TRACES: FR-PLAT-WIN-3
   # The Windows executable and its installer, cross-built from Linux
-  # (docs/windows.md §7). No Windows machine anywhere in this job: what it
+  # (docs/dev/windows.md §7). No Windows machine anywhere in this job: what it
   # can prove is that the binary links, is a Windows executable with no
   # MinGW runtime imports, starts under Wine, and that the installer installs
   # and uninstalls under Wine. What it cannot prove — a Vulkan device, a