[WIP] Switch to pydata sphinx theme

[tylerpotts]

Related to #759
This PR will change the docs to use the pydata sphinx theme

Remaining tasks:

• Navigation bar will show all subheadings when opening a document

[tylerpotts]

Current look:

[costrouc]

Thanks for adding this @tylerpotts . We definitely needed this. Just some missing content and a commented out section that needs to be removed.

[tylerpotts]

@costrouc Thanks for the review! I think I prematurely tagged you on this. I'm still working on getting the functionality/flow of the docs working and then I'll go back and add content to the pages you tagged. Will be working more on this Monday

I'll take you off as a revier for now, and add you back once it's ready to review again

[tylerpotts]

@costrouc This is ready to review. We can discuss if the version on the bottom right is still necessary. Definitely more that can be done here, but I think this is in a good place to merge

[costrouc]

Merge the source blocks but otherwise looks good!

[costrouc]

@tylerpotts the docs/requirements.txt need to be updated to include pydata_sphinx_theme

[tylerpotts]

@costrouc It's already in the requirements:
https://github.com/Quansight/qhub/blob/change_docs_theme/docs/requirements.txt#L17

Not sure how it originally got there, but that's why it's not showing in the PR