Update doc theme

[kathatherine]

Closes https://anaconda.atlassian.net/browse/DOC-498

🚧 PR under construction while I figure out if I missed any build details 🚧

But please let me know if any additions are not what we want to go for.

• Changed doc theme to pydata-sphinx-theme with custom coloring
• Added fonts that match the Anaconda docs (which massively inflates the added/changed file count)
• Added a 404 page
• Added an Anaconda favicon
• Updated the left table of contents to group pages slightly and unbury the user guide pages a little
• Suppressed some build warnings related to the sphinx.ext.autosectionlabel. See my comment in the conf.py.

New Theme Look:

[kathatherine]

@AlbertDeFusco @mattkram It appears the tests/workflows in this repo are a bit out of date. Fixing them is beyond my area of expertise. If either of y'all find the time, it would be great to get them passing so we can update the doc theme.

Happy to coordinate with one of y'all on this, in case any of my own changes end up breaking anything.

[AlbertDeFusco]

I'll find some time to look into the tests