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 Delete the superseded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/project/README.md

@@ -0,0 +1,11 @@
+# Project Documentation
+
+Architecture and project status.
+
+- [`architecture.md`](architecture.md) — the arc42-style architecture: goals, constraints,
+  building blocks, runtime views, decisions and risks.
+- [`development-status.md`](development-status.md) — the canonical implemented-features list,
+  verification baseline and known gaps.
+
+Up: [`../README.md`](../README.md) · Product plan: [`../../PLAN.md`](../../PLAN.md) · Tasks:
+[`../../TODO.md`](../../TODO.md).