[Python/Documentation] Use sphinx_rtd_theme instead of Bootstrap

[asfimport]

I have got some feedback that the Arrow Python API documentation is a bit confusing as the ToC/Menu is only really visible on the front page. People get confused by the top header. As we are already diverging from the main homepage as this was migrated to a newer bootstrap version anyway, I suggest to change the documentation theme.

As a best practice, I would switch back to the sphinx_rtd_theme as this provides a UX people are used to and happy with. We can customize it if needed later as e.g. dask did: https://github.com/dask/dask-sphinx-theme

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

PRs and other links:

• GitHub Pull Request #2776

_{Note: This issue was originally created as ARROW-3533. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Sounds good to me

[asfimport]

Antoine Pitrou / @pitrou:
+1 from me. I find navigating along the Bootstrap theme confusing.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2776
#2776