first pass at converting documentation to YAML

This is another step towards making the YAML metadata format the default, standard format. Since fdroid import creates YAML and more and more apps are using it, I think it is time to switch all the docs to use YAML instead of .txt. One open question is how much info to keep around about .txt. I'd like to see the .txt format disappear into history, and I think we're quite close to being about to do that. Anyone who wants old docs will always be able to see them in git history and archive.org.

What do you all think?

@relan @mimi89999 @Bubu @IzzySoft @vanitasvitae @licaon-kter @Rudloff