Document how to restore backup from Android Storage Access Framework

[hofbi]

https://github.com/beemdevelopment/Aegis/blob/master/FAQ.md#how-can-i-back-up-my-aegis-vault-to-the-cloud-automatically explains how to enable the backup and what is supported. But it is not fully clear how this can be restored if the cloud backup from android is used.

Open questions are:

• How can a backup from the android cloud backup be restored?
• Can I use this cloud backup to also migrate to a new phone, i.e. can I restore the backup on a new device?
• How does the D2D backup work

[vorburger]

@hofbi the 2nd commit which I've just added to #1714 explains this - hope that helps you.