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 Delete the superseded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.

tools/npc_pipeline.py

@@ -16,7 +16,7 @@ path must be absolute):
     python3 tools/npc_pipeline.py palette   # derive outfit colours -> palette.json
     python3 tools/npc_pipeline.py audit
 
-Contract (frozen in ADR-0013 and docs/blender-assets.md):
+Contract (frozen in ADR-0013 and docs/assets/blender-assets.md):
 
   * scene root "Rig" with a skinned mesh and an armature
   * bones: Head, HipL/R, KneeL/R, ShoulderL/R, ElbowL/R, HandL/R