docs(nav): stop listing "index" in content/docs/releases/meta.json (#13711) (#13946)

Fumadocs attaches a folder's index.mdx as that folder's tree `index` node only
when the folder's meta.json does NOT list "index" in `pages`. Listing it makes
the landing page an ordinary child and leaves the folder node with a name and
no url. In fumadocs-core 16.14.4 `buildFolder()`:

    if (indexPath) {
      if (excludedPaths.has(indexPath)) delete node.index;   // "index" was listed
      else excludedPaths.add(indexPath);
    }

`getBreadcrumbItems()` still emits that ancestor, but with `url: undefined`, and
`docsTrail()` in apps/docs drops any crumb without a url (Google requires `item`
on every BreadcrumbList entry but the last). The sidebar reads the same node as
`node.index ? SidebarFolderLink : SidebarFolderTrigger`.

Measured on the real loader over content/docs (405 pages), removing this one
line and changing nothing else:

  - short breadcrumb trails 8 -> 0; the 8 fixed are exactly the non-landing
    pages under /docs/releases (v9, v12..v17, implementation-status)
  - the Releases folder header goes TRIGGER -> LINK, indexUrl /docs/releases
  - /docs/releases stays in getPages() and source.getPage(['releases']) still
    resolves -- the landing page is not unlisted, it becomes the section link
  - 405 pages before and after; exactly one sidebar folder header flips and
    only the 9 /docs/releases trails change

This was the 17th and last meta.json still listing it; the other 16 were fixed
in #13710, which left this one alone because content/docs/releases/ is fenced
by AGENTS.md. This is the dedicated docs-only PR that fence names as the route.


Claude-Session: https://claude.ai/code/session_01Pk26oZ12t5N1hwGW1m1MgC

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: os-project-manager

content/docs/releases/meta.json

@@ -1,7 +1,6 @@
 {
   "title": "Releases",
   "pages": [
-    "index",
     "v17",
     "v16",
     "v15",