[Chore:]: Document release process, and update as needed

[kabir]

Our release process is a little bit clunky, and needs documenting. I think @ehsavoie mentioned a RELEASE.md at one stage.

The steps I do to do a release is:

• Run a script I have to change all the versions from e.g. 0.3.0.Beta1-SNAPSHOT to 0.3.0.Beta1
  • This only updates the POMs
  • We also have some JBang classes in the example directories which need their versions updated, which I do manually.
  • Maybe it would be better to have a script in the project, that updates everything

Then:

• Open a PR with the version update - the idea here was that the -Prelease GH Actions job will check that the PR is in order
  • But this only tests what is in the main branch already
  • And since it needs GPG signing, which needs secrets, it is set up to not run in forks
  • So the PR needs to be merged to the main branch, and the -Prelease and other jobs need to be checked that they pass (this might actually be good practice....)
• Once the above is fine, tag the release and push the tag
  • The release-to-maven-central job will pick up the tag and do all the work to push to Maven Central
  • Once the job completes it typically takes about 30 minutes for it to appear. However this varies wildly, and can take a lot longer. I think it might depend on the time of the day

Once the release-to-maven-central.yml job is completed:

• Open a PR to increment to the next SNAPSHOT version, and merge it

Also, it would be good to have the release-to-maven-central job actually publish the release to https://github.com/a2aproject/a2a-java/releases (Here is an example from another repo)