Reorganize docs into category folders and drop the retired head-review page

Group the top-level docs into docs/project, docs/design, docs/assets and
docs/process, each with its own README index. Delete the superseded
docs/screens/head-review page and sheets. Rewrite every markdown link,
absolute docs path and tool/test reference to the new locations, fix the
pre-existing broken anchors and remove the dead Joke Shop photo-board link.
check_docs OK, Python tooling 167/167.

docs/screens/refs/SIDECAR-TEMPLATE.md

@@ -34,12 +34,12 @@ attribution string for CC BY / CC BY-SA in the `Author` or a note below.
 
 | Decision | Evidence | Where it landed |
 | --- | --- | --- |
-| e.g. six-column spacing kept at 2.4 m | photo 1, measured against door width | `docs/venues.md#hollywood-boulevard` |
+| e.g. six-column spacing kept at 2.4 m | photo 1, measured against door width | `docs/design/venues.md#hollywood-boulevard` |
 | e.g. banner red sampled to `#8a1f24` | photo 1 + photo 2 | `docs/screens/refs/hollywood/` notes, facade kit |
 
 ## Generated boards used with this set
 
-Generated frames live in [`../../concepts/`](../../concepts/README.md); reference them here so the
+Generated frames live in [`../../concepts/`](../concepts/README.md); reference them here so the
 three-up sheet is reproducible.
 
 | Board | Path | Model | Prompt / seed | Generated | Spend |