Update release guide for patch version
Since releasing a patch version is a bit different than releasing a minor or major version, the release guide should be updated accordingly.
Main differences are:
- Some steps aren't necessary in a patch release (updating Maven plugins, calling for a vote etc.)
- Some branches are different too (release-xx branch already exists, and the MR are merged directly on it, we don't use the develop branch)
- Some steps are missing (explain how to merge the MR, how to cherry-pick commits if needed)
Also some information in the contribution guide should be updated:
- Developers should start from latest release branch instead of current develop branch for non API-regressive bug corrections
- changes.xml does not need to be updated when fixing this type of bug
All this to simplify the work for the releaser.