(Activities): improve Activity and AppManager documentation

What does this MR do?

Add documentation about activity sequencer (stepper).

• describe how to launch Activity for Qt application
• describe how to launch Activity for Qml application

How to test it?

make html

Associated Issues/Merge Requests

• Issues

  • Refs #5
  • Fixes Sight/sight#279

• Merge requests

  • Sight/sight!225
  • Sight/sight!207