Start using MDA theme in official repos

[lilyminium]

I expect this to require changes to the theme -- opened #32.

Repos:

• core: https://github.com/lilyminium/mdanalysis/tree/switch-docs-theme
• user guide: https://github.com/MDAnalysis/UserGuide/tree/switch-docs-theme
• mdakits registry
• mdakit cookiecutter Add theme cookiecutter-mdakit#85
• mdacli: https://github.com/MDAnalysis/mdacli/tree/switch-docs-theme
• transport-analysis
• solvation-analysis
• griddataformats: https://github.com/MDAnalysis/GridDataFormats/tree/switch-docs-theme
• hole2-mdakit
• distopia [WIP] Switch docs to new mdanalysis-sphinx-theme distopia#146
• pytng Add new mdanalysis theme + RTD on PR pytng#109
• membrane-curvature Switch to MDA rtd sphinx theme membrane-curvature#122
• mdanalysisdata
• pmda

[IAlibay]

I wanted to try it out / join in on the fun, so here's my attempt: MDAnalysis/MDAKits#61 - merged

[IAlibay]

I've opened MDAnalysis/distopia#146 for distopia.

It's unclear if they want to be under MDA branding or not, so I've handed it over to Hugo.

[lilyminium]

Thanks @IAlibay!