Add Scala Spark example doc page (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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This page should include a few examples:

1. Minimal hello-world example of running Spark locally using Mill

2. Generating an assembly jar using Mill suitable for Spark-Submit

3. One semi-realistic Spark example doing some useful computation built and run using Mill

4. One example building and running an application using PySpark

These examples should live in example/scalalib/spark/ and the documentation page docs/modules/ROOT/pages/scalalib/spark.adoc