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/identity.slint

@@ -276,7 +276,7 @@ export component IdentityScreen inherits Rectangle {
     /// existed. The same sweep, fetching the same originals; the button says
     /// what it will actually do.
     in property <bool> coverage-read-only: false;
-    // No model on disk: face indexing cannot run at all (docs/faces.md §2.2).
+    // No model on disk: face indexing cannot run at all (docs/dev/faces.md §2.2).
     in property <bool> model-missing: false;
     in property <int> picked-count: 0;
     /// Where leaving goes back to — "‹ Library" or "‹ Develop".

ui/dr-ui/ui/recovery.slint

@@ -23,7 +23,7 @@
 //
 // A restore keeps the user's collections; a rebuild cannot, because a manual
 // collection is a set of images the user assembled by hand and nothing in the
-// filesystem records it (docs/catalog.md §8.1). So the two answers are not
+// filesystem records it (docs/dev/catalog.md §8.1). So the two answers are not
 // interchangeable, the difference is stated in the button rather than in a
 // second dialogue after it, and the rebuild is the plain button even when it
 // is the only one available.

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;

ui/dr-ui/ui/toolrail.slint

@@ -17,7 +17,7 @@
 // answer the same question and only one of them is on screen at a time.
 //
 // **Why a rail is the right shape for a finger and the wrong one for a mouse.**
-// See `groups-in-rail` below, and D-N6 in `docs/ui-navigation.md` for the
+// See `groups-in-rail` below, and D-N6 in `docs/dev/ui-navigation.md` for the
 // decision it reverses and the half of that decision that still stands.
 //
 // **Why it left the chip strip.** These four used to be chips at the top of