ENH: Icon links, latest PST, more source providers, and bug fixes

[choldgraf]

This brings in the latest pydata theme, and along the way includes support for their icon-links buttons in our primary sidebar. We just inherit the component template and have some lightweight styling to make it look in-line with our documentation style.

• Brings in some more fixes from feedback in https://github.com/executablebooks/sphinx-book-theme/issues/686
• closes #285
• Fixes the bug about builder.theme_options that @mmcky reported
• closes https://github.com/executablebooks/sphinx-book-theme/issues/694
• closes https://github.com/executablebooks/sphinx-book-theme/issues/606

Also adds support for more source button providers via the pydata theme:

• closes https://github.com/executablebooks/sphinx-book-theme/issues/418
• closes https://github.com/executablebooks/sphinx-book-theme/issues/114
• closes https://github.com/executablebooks/sphinx-book-theme/issues/564

[mmcky]

Fixes the bug about builder.theme_options that @mmcky reported

thanks @choldgraf.

Once a new rc is available I will check against https://github.com/executablebooks/jupyter-book/pull/1842

[choldgraf]

OK this one now cleans up a bunch of different things, if we can get the tests passing, I'll merge so that we can cut a new release candidate since a lot of feedback was addressed and a couple new things were added.

[choldgraf]

cc @mmcky - making a new release candidate now so try it out! :-)

https://github.com/executablebooks/sphinx-book-theme/releases/tag/v1.0.0rc3