Tell contributors about the key and manual gates

CONTRIBUTING named four CI commands and the traceability tag, and
nothing about the three checks added since 0.14.1 that a first UI change
is most likely to meet: gestures-check, which fails on a key bound
without a GESTURE block or a block naming an unbound key (9d1e31f);
manual-check, which fails when index.html is not the render of the
manual (1021635); and record.sh --check, which fails when the manual
shows a picture no scene makes (70583b9). Two short paragraphs say what
each holds and where the recording tools are.

CONTRIBUTING.md

@@ -130,6 +130,24 @@ strength of plumbing a future feature would use. So: **close a requirement
 with a test that would fail if the behaviour were removed.** Coverage that
 moves slowly and means something beats coverage that moves quickly.
 
+**Keys and gestures are held the same way.** A key handler in Slint compares
+one canonical chord, `Keys.chord(event) == "Ctrl+Z"`, under a `// KEYMAP:`
+comment naming its section of the gesture book, and every key it binds must be
+named by a `GESTURE:` block beside it. `cargo run -p traceability -- gestures`
+regenerates [`docs/gestures.md`](docs/gestures.md) and the in-app help sheet
+from those blocks; `-- gestures-check` fails when a handler binds a key no tag
+names, or a tag names a key no handler binds. A `manual:` field in a block
+links the gesture to a section of the manual, and a heading that is not there
+fails the scan.
+
+**The manual is checked too.** `docs/manual/index.html` is rendered from
+`docs/manual/README.md` by `-- manual` and `-- manual-check` fails when they
+differ; `tools/manual/record.sh --check` fails when the manual shows a picture
+no scene in `tools/manual/scenes.py` makes. If you change what a pictured
+screen looks like, [`tools/manual`](tools/manual/README.md) says how to record
+it again. The pre-commit hook regenerates the matrix, the gesture book and the
+page; CI runs all three checks.
+
 ## Two invariants the build defends
 
 Worth knowing before you trip one, because both failures name a requirement