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 Delete the superseded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/build_releases.sh

@@ -3,7 +3,7 @@
 #
 # Requirements:
 #   - Godot 4.7.2 (or the version in project.godot `config/features`) on PATH,
-#     with export templates installed (see docs/RELEASING.md).
+#     with export templates installed (see docs/process/RELEASING.md).
 #   - Run from the repository root.
 #
 # Produces:
@@ -20,8 +20,8 @@ VERSION="$(grep -oP 'config/version="\K[^"]+' project.godot)"
 PLAYER_DOCS=(
   "CREDITS.md"
   "PRIVACY.md"
-  "docs/accessibility.md"
-  "docs/known-limitations.md"
+  "docs/design/accessibility.md"
+  "docs/design/known-limitations.md"
 )
 mkdir -p build
 # Presets now embed the PCK into a single binary; drop stale .pck leftovers.
@@ -95,4 +95,4 @@ shasum -a 256 build/*"${VERSION}"* > "build/SHA256SUMS-${VERSION}.txt"
 cat "build/SHA256SUMS-${VERSION}.txt"
 echo ""
 echo "Upload to: https://git.allucanget.biz/zwitschi/security-game/releases"
-echo "See docs/RELEASING.md for the full release checklist."
+echo "See docs/process/RELEASING.md for the full release checklist."