Update release documentation to separate release branch creation from release day process

[felixarntz]

As the initial release has shown (see #213), there were a few problems with the release process originally defined in #131. Most importantly, we need to change the branching strategy a bit, to properly allow for creating the release branch ahead of time, which is necessary to not block merging of other PRs ready for trunk.

That release branch was already created early for the initial release, which made sense due to the above, but then following the release instructions created some problems. Also, it shouldn't be allowed to just directly commit against the release branches, they should be protected in the same way as trunk. In other words, PRs should also have to go against the release/** branches.