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/xmp_sync.rs

@@ -2,7 +2,7 @@
 //! Standard XMP sidecars, read into the catalog and written back out of it.
 //!
 //! `dr-xmp` reads and writes the file. This is the other half the requirement
-//! asks for and the half `docs/outstanding.md` called "the larger": which
+//! asks for and the half `docs/dev/outstanding.md` called "the larger": which
 //! images a given `.xmp` describes, what the catalog holds about them, how
 //! the two are reconciled, where a disagreement goes, and the write in the
 //! other direction. Nothing here parses XML and nothing in `dr-xmp` knows a