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 Delete the superseded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/concepts/joke_shop/README.md

@@ -4,7 +4,7 @@ AI-generated art-direction frames for the fictional Joke Shop, based on the
 real Comedy Store block at 8433 Sunset Boulevard. They are generated with
 `tools/scene_concepts.py generate --group scene --only ...` and are not shipped
 game textures. The geometry mapping and source links are in
-[`../../../venues.md`](../../../venues.md#the-joke-shop).
+[`../../../venues.md`](../../../design/venues.md#the-joke-shop).
 
 ## Entrance — day, photo-anchored