Improve documentation

[mid-kid]

libmobile should be documented, focussing on easy implementation by emulators. This includes implementation requirements/expectations and tips, such as that single-thread usage is fine, but multithread is preferable, and how to synchronize the timeouts (when REONTeam/ArduinoAdapterGB#2 is implemented).

[mid-kid]

This was lightly touched upon for the v0.1 release, documenting the mobile_board_* commands, as those are by far the most implementation-sensitive. The rest is yet to be documented.

[mid-kid]

I've used doxygen for a different project, making me positive of using it as a documentation generator. Comments should be converted and a good general overview should be written up.