docs(nav): wire multi-profile-gateways guide into sidebar

Follow-up for #30240 — the new page was not referenced in sidebars.ts,
leaving it orphaned (unreachable via nav and flagged as a broken relative
link to ./profiles.md). Added under Using Hermes after profile-distributions.

website/sidebars.ts

@@ -39,6 +39,7 @@ const sidebars: SidebarsConfig = {
         'user-guide/sessions',
         'user-guide/profiles',
         'user-guide/profile-distributions',
+        'user-guide/multi-profile-gateways',
         'user-guide/git-worktrees',
         'user-guide/docker',
         'user-guide/security',