Release guide to be updated

The release guide must be deeply updated:

• The stable branch is now main, not master.
• The artifacts are now signed by the CI/CD pipeline and the signature files are pushed to Nexus at the same time as the artifacts.
• The Maven publishing portal has changed, and the new one no longer works like the old one. This has invalidated our release preparation process. Artifacts are now only pushed to our Nexus instance until the release has been validated by the PMC. Once PMC approval has been obtained, the artifacts are extracted from our Nexus instance and pushed to the Maven portal via a script using its API.
• The final tag is created after the artifacts are published on Maven Central.
• etc.