Change the pyk API docs theme

[tothtamas28]

Changes the Sphinx theme from alabaster to sphinx_rtd_theme, i.e. Read the Docs.

To check the generated docs under the new theme, run make docs in the pyk directory, then open docs/build/index.html in a web browser.

[tothtamas28]

Old theme:

[tothtamas28]

New theme:

[tothtamas28]

In case someone would like to pick another theme, here's a page that demos some themes available in Sphinx: https://sphinx-themes.readthedocs.io/en/latest/

For me, the most important aspect is how well apidoc sections can be visually separated from each other, e.g. one function signature from the next. In my opinion, Read the Docs looks pretty good in that regard: https://sphinx-themes.readthedocs.io/en/latest/sample-sites/sphinx-rtd-theme/kitchen-sink/api/.

[Baltoli]

Yep, I think this is definitely an improvement. RTD is pretty standard for Python projects in my experience