Overhaul meta-build docs

[lihaoyi]

Previous docs were kind of confusing, because they tried to demonstrate two use cases at once:

• Sharing of library configs between your build.sc and application code
• Sharing of source code between your build.sc and application code (except it never really showed the latter, so it only showed mill-build/src as a place you could put .scala files so your build.sc could use them

This PR breaks it up into two separate examples: one example illustrating how to share library configs, one example illustrating how to share source files. The original example wasn't long, but it was messy, and both new examples are much shorter and more focused on exactly what they want to show