Update documentation around gin indexes

After #3646 is merged, we should update our docs to include hints on how to improve DB performance with optional indexes.

Exact help may vary wildly based on usage (remote-settings probably doesn't need any tuning). But we can probably come up with a generic scenario.

After #3646 is merged, read the existing documentation structure and the changes it introduces. Add guidance for improving database performance with optional GIN indexes, including a generic usage scenario and noting that tuning may not be needed for remote-settings.

Written by the indexing model from the issue text.