Move "Stochastic Dynamics" section to sub-section of "Integrators"

[JIMonroe]

I'm looking at what we still need to fill in and I don't see why these should be separate. We already discuss stochastic thermostats as its own sub-section. We can still include discussion of Brownian dynamics as a special case of integrator without losing any key information, I think.

[JIMonroe]

Closing this, since it's done.