Added documentation on source file formatting.

This should get a pretty careful read-through to make sure I've correctly understood how things work. I think the stuff you helped me with today in IRC fits into the custom content type page a little better and/or the configuration page. For starters: I want people to see that "raw" content goes into the directory `sources` and there are some things which need to be considered regarding directory names, and the YAML frontmatter.

Summary of changes:

- Removed most of the content from basic-project as it is now a
  duplicate of the getting started guide. Kept the directory
  structure, will add links from this file to relevant pages as they
  are created.
- Moved frontmatter YAML information to the custom content type page.
- Heavily modified the sources file to make it more about the
  the default format for this file, and how it will be output.