Write a doc example on multi-language Mill projects (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.

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We now have support for Java, Scala, Kotlin, Python, and Typescript. We should have a page in the docs, perhaps large/multi-language.html, with an example of a multi language project:

• A Java web server
• Serving a Typescript-React frontend
• Running a Python binary as part of the backend (e.g. some ML thing?)

I expect this will take some back and forth during review, since the goal is to make a "flagship" example of a multi-language build, and so it has to not just work but also look good. But fundamentally it should not be terribly difficult

[b4s36t4]

Will work on this.