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 Delete the superseded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.

assets/models/blender_src/quaternius/README.md

@@ -21,7 +21,7 @@ Reproducing the shipped characters:
     flatpak run org.blender.Blender --background \
       --python "$PWD/tools/blender_assets.py" -- build --only npc
 
-Upstream quirks the build compensates for (see `docs/npc-model-sourcing.md`):
+Upstream quirks the build compensates for (see `docs/assets/npc-model-sourcing.md`):
 
 1. every mesh ships a **degenerate UV map** (all loops at `0,0`) with flat-colour
    materials, so the build Smart-UV-unwraps and repacks before export;