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 Delete the superseded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/tests/test_model_review.py

@@ -1,7 +1,7 @@
 """Keep the 3D model review scorecard honest.
 
 Every catalogue asset (characters, gear, props) must have a row in
-`docs/3d-model-review.md`, and the character rows must carry a verdict. This does
+`docs/assets/3d-model-review.md`, and the character rows must carry a verdict. This does
 not judge the art; it only fails when an asset is added or renamed without being
 reviewed, which is how a scorecard silently rots.
 """
@@ -13,7 +13,7 @@ from pathlib import Path
 import blender_assets as model
 
 ROOT = Path(model.__file__).resolve().parent.parent
-REVIEW = ROOT / "docs" / "3d-model-review.md"
+REVIEW = ROOT / "docs" / "assets" / "3d-model-review.md"
 
 
 def catalogue_names() -> set[str]: