Update to TOM documentation theme

[rachel3834]

I felt it was time to update the theme used for the Toolkit's documentation on ReadTheDocs.

This PR implements just the new theme and the necessary dependencies. I'm considering it preliminary because I'd like to do some reorganization of the menu structure to make it more navigable within this theme, but I thought that would be too much for a single PR. So I'm submitting this now to get some feedback

[jchate6]

This looks great!

However, I'm not a huge fan of the navigation in this format.
A horizontal nav bar doesn't lend itself well to our current list of categories.
I think if we adopt this we will want to change our tree substantially.

[rachel3834]

Thanks for your feedback @jchate6 !
Yes, I agree the navigation needs a revamp. I just wanted to get your input on the underlying theme changes before I go ahead with that. But I think this is a good chance to update the navigation in general, so that'll be my next step.