anti-feature docs

Currently we've got our official Anit-Feature explanations in our wiki. (https://f-droid.org/wiki/page/Antifeatures) I think it makes sense to move them to our website. eg. for enabling translations :)

I've copied what we've got in our wiki onto a new documentations site, and removed some technical notes which would only confuse end users.

Here's a preview of what the new anti-features doc look like: https://uniqx.gitlab.io/fdroid-website/docs/Anti-Features