Switch to PyData Sphinx theme

[speth]

Changes proposed in this pull request

• Switch to PyData Sphinx theme
• Add dropdown for switching to different doc versions

If applicable, fill in the issue number this pull request is fixing

Resolves Cantera/cantera-website#210 (I think?)

If applicable, provide an example illustrating new features this pull request is introducing

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[bryanwweber]

Resolves Cantera/cantera-website#210

This issue also encompasses the changes to the website side, too. Although pending the resolution of the content location, that may be less relevant.

[speth]

Resolves Cantera/cantera-website#210

This issue also encompasses the changes to the website side, too. Although pending the resolution of the content location, that may be less relevant.

Yeah, I realize that there's more to it, but I kind of figure that falls under Cantera/cantera-website/issues/209, which requires picking some Sphinx theme. At least this way there's no point where we have two Sphinx sites with different themes.