Use PyData Sphinx Theme

[bryanwweber]

Over the last several years, several organizations have contributed to a common theme among PyData projects. As we fit into that bucket, it'd be nice to be consistent, plus it offers some nice formatting/layout advantages.

See #209 for context for this issue. This issue shares the same non-goals as #209. It is possible that this issue will be folded into #209 as work continues there.

Action Items

• Add dependency for the pydata-sphinx-theme package
• Update the theme configuration
• Ensure content layout looks nice in the new theme, make any changes required to get that to happen
• Ensure built-in search from the theme works across the site. Supersedes Need to add a site-wide search #34
• Add version switcher only to the documentation pages