docs-audit: register the v18 landing page in the generated hand-written doc list

check:docs-audit-scope (Lint & Repo Gates) reds on any hand-written page
under content/docs that scripts/docs-audit/handwritten-docs.json does not
list. Regenerated with `node scripts/docs-audit/check-audit-scope.mjs
--write`: +1 entry, content/docs/releases/v18/index.mdx. The same one-line
companion edit the 17.7.0 release page carried.

Claude-Session: https://claude.ai/code/session_012yK1ddAdAqfzwigRHZq3r2
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

scripts/docs-audit/handwritten-docs.json

@@ -203,6 +203,7 @@
     "content/docs/releases/v17/17-6.mdx",
     "content/docs/releases/v17/17-7.mdx",
     "content/docs/releases/v17/index.mdx",
+    "content/docs/releases/v18/index.mdx",
     "content/docs/releases/v9.mdx",
     "content/docs/ui/actions.mdx",
     "content/docs/ui/apps.mdx",