New API Docs for Engines and Autopilot

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Hi @prollin and @mlaban , as mentioned here by @epaga, it would be great to have some docs on Engines API : how to start/stop all or individually, for example (I guess their status is in the Airplane State content)

Also, it would be cool to have some precise docs on how to toggle the NAV and APPR modes with the API (which commands with which params) ; and the Autopilot.GetState command seems to have been deprecated ?

You can add it to the Readme or just answer here and I'll do it

Thanks !
See you soon