docs(docs): add BacklinksList component and shared tagTarget logic for badge routing, update docs pages

apps/docs/components.ts

@@ -13,5 +13,7 @@ export default defineComponents({
   layout: {
     // Problem pages' topic + explanation badges, from `topics:` / `video:` frontmatter.
     PageHeader: "./components/ProblemBadges.astro",
+    // Topic landing + guide pages' "what links here": every problem pointing at the page.
+    PageFooter: "./components/BacklinksList.astro",
   },
 });