Improve Parser Summary Page

[WardLT]

Our parser summary is pretty ugly. We should:

• Provide richer descriptions for the purpose, output of the parsers
  • First describe the existing parsers in greater detail
• Provide quick access to the output schemas for a full description
  • First implement the schemas for existing parsers
• Display the implementors and version.

We'll probably need to write some code that injects this information into the rst documentation before building the docs. I bet Sphinx has a clever trick for that, or we could modify the Makefile.