Add documentation of new simulation config py classes

[AAndersn]

GridAPPS-D python now has classes for simulation config. Need to add these to simulation api and/or config file api sections of RTD.

from gridappsd.simulation import Simulation
from gridappsd.simulation import PowerSystemConfig
from gridappsd.simulation import SimulationConfig
from gridappsd.simulation import SimulationArgs
from gridappsd.simulation import ModelCreationConfig

Example usage here: https://github.com/GRIDAPPSD/micro-apps/blob/main/micro-apps/no-app/no_app.py#L462