Resolve duplicate content on backups

Created by: seancolsen

These two sections have some overlapping content

• https://docs.civicrm.org/sysadmin/en/latest/setup/security/#backups-and-their-security
• https://docs.civicrm.org/sysadmin/en/latest/setup/backups/

Resolve by:

1. Choose the dedicated "backups" page as the primary source of truth
2. Merge content from the security chapter into the backups chapter as needed
3. (maybe) Provide a link from the security chapter to the backups chapter. (Though I'm not sure this is necessary here.)