Add summary/definitions of various packages

[dylans]

Perhaps under the Concepts and Definitions section, or perhaps somewhere else, we should list out the various Intern packages and what they do (intern, leadfoot, digdug, and the other plugins)... unless I'm missing a place where we summarize everything that's part of Intern.

[jason0x43]

We don't have a page for that. My plan was to add a sub-page to Intern's docs, or maybe just a section in the README, that listed everything. Hmmm...or maybe the doc viewer could generate a summary page using information in its doc set config file...