Document Gazebo release name selection process

[azeey]

Summary

This documents our current process for selecting a new name for the next Gazebo release. This task is currently done by the project leader, but documenting the steps will hopefully enable other members of the PMC run it.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the feature
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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