Overhaul docs with PyData Sphinx Theme

[remrama]

This PR addresses #179 by changing the YASA docs from bootstrap theme to pydata theme. I made some other major changes along the way, mostly steps to reduce the overall size of the YASA repository. All could be undone if they are not preferred approaches.

I still want to make a few more changes, but I wonder if this PR could be merged first? I think the 5 completed tasks are probably enough to process at once here.

• New sphinx theme (pydata)
• Removed static images from repository - Docs now pull them from remote sources during build. They currently pull from the last stable YASA release, which should be acceptable for as long as necessary. Later I'd like to put the logo files in the Zenodo repository for isolated version control of any changes.
• Removed Makefiles from repository - As far as I can tell, the sphinxbuild command serves the same function and is more universal (def easier on my end). I updated the build instructions on Contributing page to match.
• Remove all build files from the repository - See next step about new workflow
• Added workflow to publish docs - This workflow is nice because it does not require a separate gh-pages branch. It requires very minimal setup.

Tasks I think should be finished in separate PRs:

• Update color scheme to match OG YASA site
• Move notebooks onto the docs site via sphinx-gallery (used by numpy, matplotlib, pandas, mne, etc.)
• Add version switcher
• Review text of all pages

The site generated from this branch can be temporarily viewed at https://remrama.github.io/yayasa (will get deleted after merge).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.24%. Comparing base (4dcd72d) to head (36df55f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #194   +/-   ##
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  Coverage   77.24%   77.24%           
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  Files          13       13           
  Lines        2659     2659           
  Branches      324      324           
=======================================
  Hits         2054     2054           
  Misses        568      568           
  Partials       37       37           

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[remrama]

The tests should be fine with the one failure. The docs workflow will fail here until some settings are changed that allow the docs to publish (it works on my fork).