docs: branded OG thumbnails with photo background

Mintlify generates a card per page (logo, section, title, description);
this puts those cards on the brand photo in dark appearance with Geist,
instead of the plain theme colours.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/docs/docs.json

@@ -37,6 +37,13 @@
 		"light": "/favicon-light-mode.png",
 		"dark": "/favicon-dark-mode.png"
 	},
+	"thumbnails": {
+		"appearance": "dark",
+		"background": "/images/og-background.jpg",
+		"fonts": {
+			"family": "Geist"
+		}
+	},
 	"fonts": {
 		"body": {
 			"family": "Geist"