Enable repository wiki

[RogueScholar]

Another aspect of the themes refresh project should be to make sure that all themes have at least one HiDPI screenshot, ideally all held in a single folder in the repository (which we can easily exclude from auto-generated archive files and tagged releases using the .gitattributes file) and rather than clutter up the repository further with a bunch of Markdown (or more to my tastes, AsciiDoc) files which display them, we could just have two or three wiki pages that showcase them, each with the images scaled to a different resolution, so interested users can quickly evaluate them at a size roughly approximating what's native to their environment and download them right from there. I'll try and kick it off with some decent documentation for contributors on keeping the Wiki orderly when adding new themes and a custom sidebar and footer once it's been turned on.

[DarthGandalf]

Contributors can update files in the repo by opening PR. Can they update wiki?