Clarify release procedure for minor releases

After the release of 1.5.1, we realized that we need to properly define the release process for a minor release. This needs to be properly documented.

The clarification should cover things like:

1. What it can and can't include (no features/bugfixes only)
2. How to publish docs? Shall we keep the current "a.b.c" version on the website or remove it and publish "a.b.c+1"?
3. Communication: possible difference in release notes, announcement, etc.