content/documentation: put docs in the tree
Gets rid of ReadTheDocs even for 5.x, and also acknowledges the existence of 6.x.
Inspired by how Knot DNS does it, however we are going to keep latest
as a proxy to GitLab Pages and possibly, when 6.1 is out, generate that
automatically from the master
branch of Knot Resolver, similarly to
the way ReadTheDocs did it.