Add documentation for ActivityPub releases

What does this MR do?

This is the documentation extracted from ActivityPub releases merge request (thread about adding this documentation).

For now, the index page is mostly taken from my design docs repository, which has also been used for the content of the epic.

Additionally, a more specific documentation has been written about how to implement an ActivityPub actor - in the limit of what has currently been implemented, the documentation will be extended with further MRs.