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.

docs/screenshots/README.md

@@ -0,0 +1,21 @@
+# Screenshots
+
+Committed gallery captures generated from the current build by src/test/capture_screenshots.gd and src/test/capture_prop_gallery.gd.
+
+## Image previews
+
+| Preview | File |
+| --- | --- |
+| <img src="01-venue-overview.png" width="280" alt="01-venue-overview.png"> | [`01-venue-overview.png`](01-venue-overview.png) |
+| <img src="02-targeting-npc.png" width="280" alt="02-targeting-npc.png"> | [`02-targeting-npc.png`](02-targeting-npc.png) |
+| <img src="03-access-inspection.png" width="280" alt="03-access-inspection.png"> | [`03-access-inspection.png`](03-access-inspection.png) |
+| <img src="04-radio-report.png" width="280" alt="04-radio-report.png"> | [`04-radio-report.png`](04-radio-report.png) |
+| <img src="05-late-night-avenue.png" width="280" alt="05-late-night-avenue.png"> | [`05-late-night-avenue.png`](05-late-night-avenue.png) |
+| <img src="06-night-flashlight.png" width="280" alt="06-night-flashlight.png"> | [`06-night-flashlight.png`](06-night-flashlight.png) |
+| <img src="07-wand-screening.png" width="280" alt="07-wand-screening.png"> | [`07-wand-screening.png`](07-wand-screening.png) |
+| <img src="08-escalation-hud.png" width="280" alt="08-escalation-hud.png"> | [`08-escalation-hud.png`](08-escalation-hud.png) |
+| <img src="09-body-camera-timeline.png" width="280" alt="09-body-camera-timeline.png"> | [`09-body-camera-timeline.png`](09-body-camera-timeline.png) |
+| <img src="10-prop-gallery.png" width="280" alt="10-prop-gallery.png"> | [`10-prop-gallery.png`](10-prop-gallery.png) |
+| <img src="11-precision-walk.png" width="280" alt="11-precision-walk.png"> | [`11-precision-walk.png`](11-precision-walk.png) |
+| <img src="12-joke-shop.png" width="280" alt="12-joke-shop.png"> | [`12-joke-shop.png`](12-joke-shop.png) |
+| <img src="13-studio-arrival.png" width="280" alt="13-studio-arrival.png"> | [`13-studio-arrival.png`](13-studio-arrival.png) |