Add docs for FollowMe plugin

[hamishwillee]

mavlink/MAVSDK#142

• Update API reference
• Add guide docs as needed
• Add doc for the new example

[hamishwillee]

All done:

• Example: https://docs.dronecore.io/v/develop/en/examples/follow_me.html
• Guide: https://docs.dronecore.io/v/develop/en/guide/follow_me.html
• Ref: https://docs.dronecore.io/v/develop/en/api_reference/classdronecore_1_1_follow_me.html