docs: add basic information about screenshots

Merged username-removed-25246 requested to merge krt/fdroid-website:docs into master

This will add some basic information on the newly landed screenshot support. I wasnt particularly involved into this, so my understanding is limited. Also I am not sure if this is the right spot, but we need to fix the whole documentation structure someday anyway ().

I would like to add same samples as well, put I want to make sure they are correct, first:

  • <upstream-git-root>/fastlane/metadata/android/en-us/phoneScreenshots/ (is fastlane part of the hierarchy?)
  • <http-server>/repo/<package-id>/en-us/phoneScreenshots/ (i think this needs to be not the http-server, but the repo directory on the local machine tha generates the index? if both, upstream and local, information exist, which gets priority?)
  • <fdroiddata>/metadata/<package-id>/en-us/phoneScreenshots/ (as far as i can tell this is not supported?)

As I said, I got my understanding from https://github.com/fastlane/fastlane/blob/1.109.0/supply/README.md#images-and-screenshots and https://gitlab.com/fdroid/fdroidserver/blob/master/fdroidserver/update.py#L554 , so it would be nice to have @eighthave or whoever worked on this comment here.

Also it would be nice if we plan to follow fastlane for other stuff like short description, changelogs etc. as well.