Releases: add mermaid Gantt chart

[scpeters]

🎉 New feature

Summary

This adds a Gantt chart for the Gazebo collection release and EOL dates using mermaid-js syntax (experiment at https://mermaid.live), which is supported by GitHub.

It renders fine in GitHub, though it could be styled better (colors and font sizes), but I'm leaving this in draft until confirming that our sphinx backend will render it properly. I have successfully tested this locally.

Test it

Preview on GitHub

Checklist

• Signed all commits for DCO
• 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)
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[azeey]

You'll need to add sphinxcontrib-mermaid to requirements.txt. I started working on on a timeline for the release process, but I abandoned the idea. Here's some WIP code that might be useful master...azeey:docs:mermaid

[scpeters]

thanks! I incorporated some of those changes in c0fe9ca and successfully tested it locally