Update all docs following changes for more flexible plugin architecture

[hamishwillee]

mavlink/MAVSDK#188

This will be potentially a lot of work. Start next week.

[hamishwillee]

Bunch of changes
#93
#88
#86
#94
#95

[hamishwillee]

I think this is done now.

There are possibly some issues in plugins, and there are some missing API ref files like en/api_reference/structdronecore_1_1_device_1_1_mavlink_handler_table_entry.md

However Julian says that this area will be changing in the coming weeks so not worth updating further right now.