Add script for bulk-clearing room retention policies in the admin FAQ

[Johennes]

Description:

@babolivier has a nice script for pruning retention policies here that can be useful in other contexts, too (e.g. when bulk-changing room history visibility settings). It might be good to include this in the "Admin FAQ" section of the docs.

Putting this here as an issue for discussion first in case somebody else knows a better place to host this.