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.

tools/bench/src/exporting.rs

@@ -16,7 +16,7 @@
 //!
 //! That is a deliberate refusal rather than an oversight. `CONTRIBUTING.md`
 //! asks that a requirement be closed by a test that would fail if the
-//! behaviour were removed, and `docs/code-health.md` CH-4 records what the
+//! behaviour were removed, and `docs/dev/code-health.md` CH-4 records what the
 //! coverage figure looks like when tags are hung on plumbing instead. A tag
 //! here would say the export budget is checked; the GPU half of it would still
 //! be unchecked.
@@ -26,7 +26,7 @@
 //! It is a **one-sided** gate, and that is worth having. The encode half is a
 //! lower bound on the whole: if resizing, sharpening and encoding 24 MP alone
 //! take longer than two seconds, NFR-P7 is violated no matter how fast the
-//! render is. So the budget in `docs/bench-baseline.json` is the requirement's
+//! render is. So the budget in `docs/dev/bench-baseline.json` is the requirement's
 //! own 2000 ms, and exceeding it fails the build honestly. Coming in under it
 //! proves nothing about the requirement, and the report says so rather than
 //! printing a tick.
@@ -64,7 +64,7 @@ const RUNS: usize = 5;
 
 /// A row of the export table.
 pub struct EncodeRun {
-    /// The name this row carries in `docs/bench-baseline.json`.
+    /// The name this row carries in `docs/dev/bench-baseline.json`.
     pub key: &'static str,
     pub label: &'static str,
     pub width: u32,