Add scaladoc for the FSM lib based on RTD doc

[mrcmry]

I used the SpinalDoc-RTD to add scaladoc the the state machine library.

This way a developer can quickly access the doc when coding.

Still open but not blocking for PR:

• StateMachineSlave is not documented
• the method innerFsm is not used anywhere (at least in this repos code). It's a bit confusing, because some example use classes or variables using this name. Perhaps we should remote it or deprecate it.
• I added a scaladoc description as "used internally" for getStateMachineAccessor whenIsActiveWithPriority. Perhaps we should make them protected or private, but it could break things, I don't know

Impact on code generation

Do not change the scala code an thus the generation as well.

[Dolu1990]

Thanks ^^