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/baseline.rs

@@ -1,6 +1,6 @@
 //! The committed numbers, and what counts as a regression against them.
 //!
-//! `docs/frame-budget.md` commits its measurements by hand and says why: *"a
+//! `docs/dev/frame-budget.md` commits its measurements by hand and says why: *"a
 //! regression should be a diff rather than somebody's memory."* This is the
 //! same idea in a form a program can read, because §8 asks for more than a
 //! record — *"a regression beyond stated tolerance fails the build"*.
@@ -80,7 +80,7 @@ pub struct Metric {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct Baseline {
     /// Prose for whoever opens the JSON first. Kept in the file rather than
-    /// only in `docs/benchmarks.md`, because the person who finds this in a
+    /// only in `docs/dev/benchmarks.md`, because the person who finds this in a
     /// failing CI log is not reading the docs directory at that moment.
     #[serde(rename = "_readme")]
     pub readme: Vec<String>,
@@ -173,15 +173,15 @@ impl Baseline {
 
     /// Where the committed baseline lives, found the way `tools/traceability`
     /// finds the repo root: by walking up from this crate's manifest until
-    /// `docs/requirements.md` appears.
+    /// `docs/dev/requirements.md` appears.
     pub fn default_path() -> Result<PathBuf> {
         let mut dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
-        while !dir.join("docs/requirements.md").exists() {
+        while !dir.join("docs/dev/requirements.md").exists() {
             if !dir.pop() {
                 anyhow::bail!("could not locate the repo root above this crate");
             }
         }
-        Ok(dir.join("docs/bench-baseline.json"))
+        Ok(dir.join("docs/dev/bench-baseline.json"))
     }
 }