Add documentation folder READMEs and visual previews

Create an index README in docs/ and every tracked docs subfolder, with relative
image galleries for Blender previews, screen designs, screenshots and concepts.
Generated realism-run READMEs were also created on disk but remain gitignored.
Validate all Markdown/image links.

README.md

@@ -106,6 +106,7 @@ Full setup, every run/test/smoke command, and the controls table are in
 - [`DONE.md`](DONE.md) — Completed task sections.
 - [`MEMORY.md`](MEMORY.md) — Persistent session notes.
 - [`CHANGELOG.md`](CHANGELOG.md) — User-facing changes by version.
+- [`docs/README.md`](docs/README.md) — Documentation index and visual previews.
 - [`docs/architecture.md`](docs/architecture.md) — arc42-style architecture documentation.
 - [`docs/gameplay.md`](docs/gameplay.md) — Gameplay systems, venues, and the escalation model.
 - [`docs/tutorial.md`](docs/tutorial.md) — First-shift tutorial beats and rules.