Updates to 'rendering output' chapter

[cpsievert]

1. Add new serializers to the table on https://www.rplumber.io/docs/rendering-and-output.html#serializers

2. Add at least one example of a custom serializer. If not a real example, perhaps just copy an implementation of an exported serializer?

3. Add a simple description of the main aspects of the response object (i.e., body, res) as well as a pointer to where folks can learn more (i.e., rook documentation)

4. In section 4.2.3, I think

#' @png (width = 400, height = 500)

Should be

#' @png list(width = 400, height = 500)

[schloerke]

In section 4.2.3, I think

This is inconsistent plumb() behavior. @IMAGETYPE(arg1..., arg2...) is valid syntax. Other serializers use the @serializer SERIALIZERNAME list(arg1..., arg2...) syntax.

[cpsievert]

Oh, right, makes sense. That seems worth explaining as well, maybe just under the table of serializers