ADD ActivityPub blueprint

What does this MR do and why?

This is the initial document for blueprints related to ActivityPub features.

the "Description of the related tech and terms" is what was already in the initial design doc repos, the epic and the documentation as the "What?" section, with possible wording changes from technical review but no change in meaning, so you can skip them if you're already familiar with them. Similarly, the "Motivation" section is a paste from the "Why?" section.

"Proposal" reflects the "How" section, but there are some changes related to the search feature, so it will need your attention.

The "Summary" and "Design and implementation details" are entirely new (although the later points take their text from the related issues). I have documented in details what has already been done ("Endpoints" and "Serializers", already implemented for the releases actor) and what I'm currently doing ("Subscriptions"), we'll fill the rest in iterations, if that's fine with you.