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 Delete the superseded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/blender_assets.py

@@ -4,7 +4,7 @@
 Everything here is modelled procedurally inside Blender (flatpak, headless) so
 the assets are reproducible from source. Outputs are staged for *later*
 integration: nothing in the game loads them yet (see
-`docs/blender-assets.md` for the integration contract).
+`docs/assets/blender-assets.md` for the integration contract).
 
     # build every asset (or --only name,name) -> assets/models/blender/**.glb
     flatpak run org.blender.Blender --background \
@@ -1489,7 +1489,7 @@ def build_character(name, head="face"):
 # is 4 skinned meshes on the same 62-joint armature; the build joins them
 # **without welding** (a weld corrupts the skin weights), renames the upstream
 # materials onto the five tint slots, unwraps the degenerate source UVs and bakes
-# the arm-down rest. See docs/npc-model-sourcing.md for the measurements.
+# the arm-down rest. See docs/assets/npc-model-sourcing.md for the measurements.
 
 QUATERNIUS_SRC = SRC_DIR / "quaternius"
 
@@ -1497,7 +1497,7 @@ QUATERNIUS_SRC = SRC_DIR / "quaternius"
 # (face, hair detail and hivis fabrics); anything not listed is kept too, so a
 # new upstream material shows up as an extra fixed-colour slot instead of
 # silently disappearing. The maps were derived from the per-material height bands
-# measured in docs/npc-model-sourcing.md.
+# measured in docs/assets/npc-model-sourcing.md.
 QUATERNIUS_SOURCES = {
     "casual": {
         "file": "Casual.glb",
@@ -2605,7 +2605,7 @@ def build_gear(name):
         # procedural library's 0.315 m, and its centre sits 0.07 m higher, so the
         # head gear is scaled and lifted to match. The procedural fallback then
         # wears slightly small hats — cosmetic, and recorded in
-        # docs/blender-assets.md.
+        # docs/assets/blender-assets.md.
         centre = Vector((0.0, 0.0, HEAD_GEAR_LIFT))
         obj.data.transform(
             Matrix.Translation(centre)