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 Delete the superseded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.

PRIVACY.md

@@ -14,7 +14,7 @@ Player data is written only to Godot's local `user://` directory:
 
 Those files stay on the player's machine unless the player copies them. Deleting
 the files (or the `Gig Line` user directory) resets the profile. See
-[`docs/getting-started.md`](docs/getting-started.md#save-data-and-migration) for
+[`docs/process/getting-started.md`](docs/process/getting-started.md#save-data) for
 locations, migration and rollback instructions.
 
 The repository's development tools can call OpenRouter when a developer supplies