Suggest deleting backup file transferred to server after restoring from it

As part of QA, the QA team transfers a backup .zip file to the QA server and restores from it. However, they haven't always deleted the file afterwards, which made me realize that deleting the file isn't one of the instructions at https://docs.getodk.org/central-backup/#restoring-a-backup. I think it'd useful to suggest that, because the user might not know to do it, and the file can be very large.

Open the restoring-a-backup section at https://docs.getodk.org/central-backup/#restoring-a-backup and review its existing instructions. Add a suggestion to delete the transferred backup file after a successful restore, then verify the documentation builds and the instruction is clear.

Written by the indexing model from the issue text.