Add `Field` and `Argument` types to paledoc

[rknLA]

For the basics/builtins, these should be pretty straight forward, but since folks will probably implement their own field types and argument types, it probably makes sense to have documentation renderings of the description of the argument outside of the context of how it's used (i.e. right now we have IntegerArgument included with its description within the endpoint doc, but we don't have a general "this argument represents an integer" doc.)