Use a Jekyll collection for apps

https://jekyllrb.com/docs/collections/

Looks like for each app, we would want _one_ yaml/md file. Hence we should probably combine all input files (e.g., desktop files, AppStream metainfo, etc.) into _one_ yaml/md/json file per application.

This will have the added benefit that external consumers (e.g., app centers) will be able to easily use the same file too.