Create docs for each class

[rcannood]

@lazappi already filled in some documentation for the classes.

Let's remove the @noRd in each (/most?) of the Rd classes.

Of course, the classes won't have all of the fields that the user will want to use. For instance, in the documentation of Instance it should be noted that an instance will contain the core data structures, e.g. db$Artifact, and that these are objects of class Registry.

[rcannood]

Implemented in #36