Document Camera Interface: MAVLink Camera Protocol

[hamishwillee]

A high level document about the MAVLink camera interface.

• This should link to the official docs but not try and reproduce them: https://mavlink.io/en/protocol/camera.html
• Explain that PX4 can integrate with cameras that support the MAVLink camera protocol, and that the CSD implements this protocol for any connected camera (essentially it provides an adaption layer for creating a MAVLink interface)
• Explain the level of support for the API.
• Show message flow at high level for both hardware and simulation case.

As part of the task the MAVLink Protocol document should be moved from the wiki into /docs/en/interface_mavlink_camera_protocol.md.

Migrated from Dronecode/camera-manager#102

[hamishwillee]

Level of support for APIs is covered in overview

I am not "so" convinced this topic is needed, but it might be useful to have a document like the one for RTSP - #11 showing the broad architecture/flow.