Add tentative Release timeline for Kura

[azeey]

Summary

The timeline is based on what we have done in the past releases (e.g. gazebo-tooling/release-tools#1271), but shifted one month earlier so as to avoid overlap with ROSCon.

This also adds detail about each of the releases phases which haven't been documented well, as far I can tell.

Checklist

• Signed all commits for DCO
• Added tests
• 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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Generated-by: Gemini 2.5 Flash

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[scpeters]

minor comment about wording