Overhaul docs for The Mill Evaluation Model

[lihaoyi]

Renovated it to freshen it up: diagrams, cross-links, and general cleanup. Also renamed it The Mill Execution Model to avoid confusion with the Evaluation phase in Mill

Fixes #4114

[asarkar]

I'm not seeing these changes here: https://mill-build.org/mill/depth/evaluation-model.html, has it been published yet? Perhaps, a last updated date will be helpful to have in the docs.

[lihaoyi]

@asarkar the updated docs will go out with the next release, for now you can see them pre-release at https://mill-build.org/mill/main-branch/depth/execution-model.html