Clean up environment file to mitigate sphinx version problem

[kafitzgerald]

Clean up environment file to mitigate sphinx version problem:

• Removes old sphinx theme from the environment file and related template file
• Removes jupyter and sphinx from the environment file so they're just installed as dependencies (hopefully this mitigates the sphinx version issue we are having)

Closes #541.

[kafitzgerald]

Ok, so now this is just failing because of the matplotlib update as logged in NCAR/geocat-viz#160.

[kafitzgerald]

Unfortunately, the ReadTheDocs build is just going to be failing for a bit here. I did verify that it's back to the "Command killed due to timeout or excessive memory consumption" mode of failure.

My take is that we should start merging some things as long as we're confident that everything else is passing and the RTD failure is for the known reason and I can see if there's another potential interim solution (given that we are keeping the notebooks for a bit per #524 and current timelines).