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/ui/settings.slint

@@ -98,7 +98,7 @@ export component SettingsPage inherits Rectangle {
     /// it — so both buttons go quiet together.
     callback reindex-faces();
     /// TRACES: FR-CULL-8
-    /// Which detector the pass finds faces with — docs/faces.md §12.3 for
+    /// Which detector the pass finds faces with — docs/dev/faces.md §12.3 for
     /// what each costs and finds. The choice is a model change: coverage is
     /// counted per pipeline, so picking another one starts from nothing.
     in property <[string]> face-detector-labels;
@@ -144,7 +144,7 @@ export component SettingsPage inherits Rectangle {
     in property <string> adapter;
     in property <string> backend;
     /// What runs the neural models and how it was chosen — the two lines
-    /// `dr_ui::inference::about_lines` produces (docs/inference.md §4).
+    /// `dr_ui::inference::about_lines` produces (docs/dev/inference.md §4).
     in property <string> inference-backend;
     in property <string> inference-detail;
     in property <int> fps;