improved documentation on release procedure, merge type for dev to main

[shimwell]

Description

This tiny fix to the docs adds details of the merge type that should be used when PR from dev to main. We tried the rebase PR last time and somehow it missed 25 commits so we now think the "create a merge commit" is the way to go. Sorry for my mistake

Checklist

• [x] I'm making a PR from a feature branch on my fork into GEOUNED-org/GEOUNED/dev branch
• [ ] I have followed PEP8 style guide for Python or run a formatter such as black or ruff format on my code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works