Add documentation on colormaps

[TomDLT]

As suggested in https://github.com/gallantlab/pycortex/pull/395#issuecomment-870054614, we could have some section on the pycortex documentation page that organizes the colormaps somehow, maybe something like how they give lists of "perceptually uniform", "sequential", "diverging", "qualitative" etc. colormaps in matplotlib, or at least describing a small number of good default choices.

I think it would be particularly nice for 2D colormaps.

[tknapen]

Just chiming in; I've made my own png files to serve as colormaps in Pycortex, perhaps that path could be described too?