Change website theme

[coala-bot]

Though it looks comfy, headers and paragraphs (especially the overview in the sidebar) are hardly readable and distinguishable, we should change that^^

Opened via gitter by @Makman2

[Mariatta]

I guess somewhat related to https://github.com/coala/documentation/issues/293

Do we want to revert back to sphinx_rtd_theme ? Maybe agogo theme could work too, I think that's what Python official docs use.

I have some time later this evening, I can try experimenting with these two themes.

[Makman2]

@Mariatta cool :D :+1: