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.

core/dr-pipeline/src/operation.rs

@@ -54,7 +54,7 @@ pub enum Affects {
     /// A pixel's *neighbourhood* — sharpening, noise reduction, clarity,
     /// texture, dehaze, spot removal.
     ///
-    /// The seam `docs/requirements.md` §3.3 designed and nothing cut until
+    /// The seam `docs/dev/requirements.md` §3.3 designed and nothing cut until
     /// [`crate::detail`] existed. It is a separate variant rather than a flavour
     /// of `Colour` because it is a separate *dispatch*: a fragment in the fused
     /// pass is handed a colour and has no way back to a coordinate, so a