Add gimbal docs

[hamishwillee]

The API went in as mavlink/MAVSDK#124 and API Reference is now present in the develop tree docs here.

• The API is still missing telemetry to get gimbal position
  • delivered in Add Gimbal telemetry MAVSDK#141 (in telemetry)

I don't think there is justification for a gimbal guide doc. Propose a "Camera and Gimbal" doc that explains what options the API provides for camera control. I suspect just missions, with gimbal setting the gimbal behaviour during mission.

[hamishwillee]

Also track mavlink/MAVSDK#154 for this task