Documentation for PyProject.toml

I'm working on creating some CircuitPython libraries, so I was very happy to see #188 get merged in. However, documentation around this new feature is missing and others may not be aware of it's recent inclusion.

Start by reviewing the feature introduced in issue #188 and the repository's existing documentation structure. Document how CircuitPython library authors use PyProject.toml with this feature, and consider the documentation complete when the new capability and its usage are clear to someone creating a library.

Written by the indexing model from the issue text.