RFC 0072 FCP, Switch to CommonMark for documentation

Is an example with instructions for how to convert a section from Docbook to CommonMark. I haven’t been recommending using a tool to do the conversion since people need to manually change some things anyway.

I’ve been trying to add these instructions to the conversion project for each section so people can do the work in bitesize pieces:

https://github.com/NixOS/nixpkgs/projects/37