Document `content_dir` and `layouts_dir`

This is a potential fix for nanoc/nanoc#488.

(I’m not entirely happy about the structure of the _config.md_ file. I think it should list default values explicitly, and have an example for every case. It also makes sense to have a separate section only for the *filesystem* data source, I think.)

CC @jm3 @gpakosz 
Oops, thanks for spotting that.