change style of docs to UserGuide style

[orbeckst]

Is your feature request related to a problem? Please describe.
The generated sphinx docs https://www.mdanalysis.org/docs/ are based on the alabaster style with modifications but the overall look&feel of the website is different. The styles should match better.

Describe the solution you'd like
@lilyminium solved this problem with the style of the UserGuide https://www.mdanalysis.org/UserGuide/ . We should use her style for the docs for a more uniform look & feel.

[orbeckst]

I think all @MDAnalysis/coredevs are happy to change the style of the docs!

@lilyminium could you please look into it? (This will also be an immediate and very visible impact of GSoD.)

[orbeckst]

I am particularly interested in the commit history to see what needs to be done so that we can do the same things for our other projects...

[orbeckst]

See https://www.mdanalysis.org/mdanalysis/ — looks great on desktop and mobile!