Auto-generate documentation for config parser

rapidpe_rift_pipe.config.Config should be the single source of truth for configuration options. Right now, however, the options it supports are not documented outside of source code comments. As an improvement, I propose the following:

• Modify how the Config class is set up, so that each variable has an associated static doc string
• Add a static method to output summary info (perhaps just by modifying __doc__)
• Have Sphinx call this method in order to display this documentation on a webpage (depends on #52 (closed))