Documentation using DocC

[nighthawk]

Would be good to add documentation for this package, using DocC.

This is blocked currently as this package is primarily a collection of extensions and DocC doesn't yet support those. This is being worked on though: https://forums.swift.org/t/document-extensions-to-external-types-using-docc/56060

Once that's implemented, it'd be great to then add the docs to SPI.