Document overview and UI workflow for Update Control

Depends on Ui design being complete.

Acceptance criteria:

• Overview -> State script is renamed to Overview -> Customize the update process
  • Covers existing state script content plus the new Device-side API and Synchronized update
  • Screenshots of deployment dialog with pause functionality highlighted.
• List some use cases and describe how it works. Explain when to use State scripts vs. Device-side API
  • "If it's important to adhere to a maintenance window, you can stop the device before installing, etc. etc."
• Device-side API defined in Overview -> Taxonomy
• See Mender 2.8 documentation architecture proposal for more information