Add @seealso and links to other pages

[perrydv]

I'm thinking we should go through roxygen entries to add more cross-referencing via @Seealso and \code{\link{otherRelevantEntry}}. E.g. right now help(modelBaseClass) is noted in the @details of help(nimbleModel) but it would help to make that a \link and to put it in @Seealso. I'm putting this is a nimble-docs issue just to keep it separate from code issues.