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 Delete the superseded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/README.md

@@ -15,13 +15,13 @@ The map-optimization references for the El Capitan Entertainment Centre block
 (6840 Hollywood Blvd) are in
 [`hollywood_boulevard/README.md`](hollywood_boulevard/README.md). The real-site
 facts and game mapping are documented in
-[`../../venues.md`](../../venues.md#hollywood-boulevard).
+[`../../venues.md`](../../design/venues.md#hollywood-boulevard).
 
 ## The Joke Shop — Sunset Strip
 
 The current Joke Shop map is based on The Comedy Store at 8433 Sunset Blvd.
 The real-site mapping and simplifications are in
-[`../../venues.md`](../../venues.md#the-joke-shop); the frames are in
+[`../../venues.md`](../../design/venues.md#the-joke-shop); the frames are in
 [`joke_shop/README.md`](joke_shop/README.md).
 
 ![Joke Shop entrance in daylight](joke_shop/joke_shop_entrance_day.png)