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.