Docs: Document `remote_cache` is only used if a remote database is used (#104779)
* Document `remote_cache` is only used if a remote database is used
* Update _index.md
---------
(cherry picked from commit 19a65a4865)
Co-authored-by: Mathias Petermann <mathias.petermann@gmail.com>
Co-authored-by: Jacob Valdez <jacob.valdez@grafana.com>