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 Delete the superseded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/hollywood/README.md

@@ -11,7 +11,7 @@ TV Studio board.
 
 Provenance, licences and the decisions these photos informed are in
 [`SIDECAR.md`](SIDECAR.md). The rules for this folder are in
-[`../README.md`](../README.md); generated art-direction frames live in
+[`../README.md`](README.md); generated art-direction frames live in
 [`../../concepts/hollywood_boulevard/`](../../concepts/hollywood_boulevard/README.md).
 
 | Photo | What it is evidence for |
@@ -35,5 +35,5 @@ Provenance, licences and the decisions these photos informed are in
 - The east retail neighbour beyond the temple (the modern retail block), for facade rhythm at the
   right-hand edge of the venue.
 
-Add new files with the naming convention in [`../README.md`](../README.md) and list them in the
+Add new files with the naming convention in [`../README.md`](README.md) and list them in the
 sidecar in the same commit.