Re-license documentation as CC-BY

At the moment, our documentation is licensed as AGPL like pleroma itself. This can be a problem for people who run wikis or blogs and don't want to put their whole writings under the AGPL too. Let's relicense under CC-BY, which should be good enough for our purposes but also make it usable essentially everywhere.

Steps for this

1. Find all the people who contributed to the docs
2. Contact them and get them to make a commit confirming that they agree to license their doc contributions under CC-BY
3. If we can't find them all or get them all to do it, rewrite the parts that are left