Add state diagram for compute

[tristan957]

Models a compute's lifetime.

Here is a link to what it looks like rendered. Or use the "Rich Diff" on GitHub for viewing the diagram

[tristan957]

This is going to be useful for some of the changes coming out of the pg_upgrade work.

[github-actions]

2706 tests run: 2574 passed, 0 failed, 132 skipped (full report)

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Code coverage* (full report)

• functions: 28.3% (7136 of 25210 functions)
• lines: 46.9% (43718 of 93293 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
035117f at 2024-03-20T20:35:05.265Z :recycle:}

[hlinnaka]

I suggest putting this to compute_tools/README.md. That way it's closer to the code that it describes