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 Delete the superseded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.

docs/screenshots/README.md

@@ -29,6 +29,6 @@ README, the gameplay docs and the screenshot gate; do not edit them by hand.
 | 21 | [Responder arrival](21-responder-arrival.png) | A responder at the locked door |
 
 The captures are 1280×720 PNGs. Reference designs and the judge report live in
-[`../screens/`](../screens/README.md) and [`../ui-copy.md#screen-design-judge-report`](../ui-copy.md#screen-design-judge-report);
+[`../screens/`](../screens/README.md) and [`../ui-copy.md#screen-design-judge-report`](../design/ui-copy.md#screen-design-judge-report);
 the re-evaluation's pose and review sheets are under
 [`../blender_previews/`](../blender_previews/README.md).