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.

ui/dr-ui/src/library.rs

@@ -4261,7 +4261,7 @@ pub fn thumbs_dir(account: &Account) -> PathBuf {
 /// **Not shipped with the application** and not a build input: the InsightFace
 /// weights carry a non-commercial research grant incompatible with this
 /// project's licence, so the user obtains them and the app loads them from here
-/// (docs/faces.md §2). An absent directory is the ordinary state of a fresh
+/// (docs/dev/faces.md §2). An absent directory is the ordinary state of a fresh
 /// install, not an error.
 pub fn face_models_dir(account: &Account) -> PathBuf {
     catalog_path(account)
@@ -4334,7 +4334,7 @@ impl FaceModelPaths {
 }
 
 /// Where the inference engine keeps what it derives per device: the probe
-/// result and compiled engines (docs/inference.md §4, §5). A peer of
+/// result and compiled engines (docs/dev/inference.md §4, §5). A peer of
 /// `thumbs`, not of the catalog: disposable, regenerable, never synced.
 pub fn inference_cache_dir() -> PathBuf {
     data_root().join("inference")