feat: crumb to the parent and read forward

A part page's parent is its last crumb and is marked in the sidebar.
A Next steps link pointing back in the reading run gives way to the run
order, so Start here reads every page once.

website/src/content.config.ts

@@ -19,6 +19,9 @@ export const collections = {
 				// A mark drawn beside the title (a platform's logo), a path under
 				// the site root such as assets/logos/batocera.svg.
 				logo: z.string().optional(),
+				// The page a part page belongs to (a console of a setup's list),
+				// read by routeData.ts: the last crumb, marked in the sidebar.
+				parent: z.object({ label: z.string(), link: z.string() }).optional(),
 			}),
 		}),
 	}),