sites: Add support for readthedocs.io

[Holzhaus]

Add some fixes for readthedocs.io.

Screenshots (Gruvbox):

[alphapapa]

Hi Jan,

Thanks also for this PR. A few notes:

1. In the screenshot, could you improve the contrast of the Configuration File line in the nav sidebar?
2. Below that, the Version 2 and Version 1 are the same background color as the rest of the sidebar. Is that how it is in the default RtD theme, or should those sub-headings have a different background color? Not that we have to follow it exactly, just a thought.
3. The CSS selectors are very specific and verbose. This isn't necessarily bad, but it might be preferable to make them as simple as possible. For example, instead of .rst-content .wy-alert-info.admonition .wy-alert-title, would .wy-alert-title be sufficient?
4. I also made a line review comment about a possible missing comma.

Thanks.

[alphapapa]

Is a comma missing at the end of this line?

Or, rather, are any of these commas necessary? I don't think they are, since it's Stylus.