documentation related to code that was moved out of extras into packages

[ViralBShah]

Our documentation continues to have documentation related to code that used to be in extras, but now has been moved to packages. Perhaps the relevant documentation should also be moved to the packages. eg. ArgParse.

It would be nice if all package documentation was accessible in a standard way.

[binarybana]

Per-package documentation (among other things) is being discussed in #1619.

[JeffBezanson]

Removed by 20db526.