Switch sphinx book theme

[NickleDave]

Redone as described in #74

[NickleDave]

Build from PR is here:
https://dev-review--75.org.readthedocs.build/en/75/

[bielsnohr]

Hi @NickleDave ! Thanks for the great work on this, and sorry it took so long to get around to looking at it. I think overall, the site looks really good in this format. A few additional things to consider:

• I think we probably should change the name of the readthedocs site, but perhaps lets address that in a separate issue and PR.
• There are a lot of warnings spit out by Sphinx, but the site does build. Again, probably something to address is a separate issue and PR. Happy to raise one.
• We need a better navigation bar on the side that allows for navigation throughout the site. Does sphinx-book-theme have this capability?

[NickleDave]

@bielsnohr I think this addresses all your requested changes.

Any in-line request that you mentioned in comments I raised separate issues for.
If you all agree, I'd prefer to get this merged in and re-work from that base, instead of iterating forever on this branch.

[bielsnohr]

Thanks for addressing my comments. Happy to have this merged and then address other bigger items in the separate issues that @NickleDave has raised.