Flesh out Evaluator example docs (500USD Bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

See https://github.com/orgs/com-lihaoyi/discussions/6 for other bounties and the terms and conditions that bounties operate under

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We previously only had one tiny hard-to-find section on the docs briefly mentioning Evaluator Commands https://mill-build.org/mill/fundamentals/bundled-libraries.html#_evaluator_experimental. Now that #4094 has landed, we should flesh it out with example use cases of resolveSegments, resolveTasks, plan, execute, and evaluate. These should be sufficiently realistic and sufficiently documented that someone reading the examples can figure out how to use Evaluator Commands in their own projects to do useful things.

This exercise might also flesh out some missing pieces or idiosyncrasies of the Evaluator API, allowing us to fix them before the API is frozen by backwards compatibility requirements in 0.13.0

These example docs should be promoted to a standalone page in the documentation under website/docs/modules/ROOT/pages/extending/