DCB: UI Documentation

Some documentation restructuring is necessary as well, due to NMS-14013, which was written first and provided the non-UI documentation.

Location: Operation>Device Configuration Backup

1. Update introduction.adoc.
   1. Modify current content: update introductory text to provide reason why users want to backup device configuration/value prop (disaster recovery, etc.).
      1. include basic workflow with links to the UI section.
   2. Create "Create Device Configuration Backup" topic and provide step-by-step instructions on how to do this.
   3. Describe any additional UI-related features (events/alarms for failed backups, works with Minions, etc.)
   4. Document options: View History, Download, Backup Now, Compare. Does not need to be step-by-step, as UI is pretty intuitive, but describe how compare works, as it might not be clear.