feat(docs): add changelog page and switch tag guide paths to appendix

apps/docs/topic-landings.ts

@@ -4,10 +4,10 @@
  * tag", read by the sides that must agree:
  *
  * - `components/ProblemBadges.astro` links a covered tag's badge to its
- *   landing page, and every other tag's badge to `/guides/<slug>`;
+ *   landing page, and every other tag's badge to its Appendix guide, `/appendix/<slug>`;
  * - `components/BacklinksList.astro` lists, under a landing or guide page,
  *   every problem whose badge links there — both via `tagTarget()`;
- * - `apps/cli/sync.ts` creates a placeholder `content/guides/<slug>.mdx` for
+ * - `apps/cli/sync.ts` creates a placeholder `content/appendix/<slug>.mdx` for
  *   every tag a problem page carries that is NOT a key here.
  *
  * Add a key when a campaign topic starts teaching a tag; its guide page is then
@@ -61,7 +61,7 @@ export function tagTarget(
   const prefer = page.tags.some((t) => t.slug === "tree") ? "/tree-" : "/graph-";
   const landing = landings.includes(own) ? own : (landings.find((p) => p.startsWith(prefer)) ?? landings[0]);
   if (landing) return { href: landing, kind: "landing" };
-  const guide = `/guides/${tag.slug}`;
+  const guide = `/appendix/${tag.slug}`;
   if (served.has(guide)) return { href: guide, kind: "guide" };
   return { href: `https://leetcode.com/problem-list/${tag.slug}/`, kind: "external" };
 }