Generate documentation for the YAML spec

[aaronsky]

Is your feature request related to a problem? Please describe.
Right now, docs/configuration.md is manually maintained by me. This takes up time I could be using to do something else, and also introduces inconsistencies between the documented configuration structure and the actual structure.

Describe the solution you'd like
I'd like this file to be generated to be based on the source code structure and code comments. I'd also be interested in exploring non-YAML ways of conveying the structure of this file, such as a header hierarchy like what's used in XcodeGen's Project Spec document. I'd like it to be generated during the Github Pages build, so Jekyll scripting will need to be involved somewhere without breaking Pages support.