Restyle Jupyter Book

[orifox]

I'd like to see if we can make the Jupyter book page look more like STScI's other Readthedocs.
Here is an example comparison: https://jwst-pipeline.readthedocs.io/en/latest/jwst/introduction.html

Some styles that are noteworthy:
*blue box with STScI logo in upper left
*grey backgrounds on left/right sides, but white in the middle
*overall font/size choice for both main text and side bar
*It would still be good if we could figure out how to make the TOC collapsable. I know it's possible, I just haven't been able to make the code work yet