Add optional point argument to AddDocumentLink

[arrgh1]

I was adding table of contents to a generated document and realized that PDFSharp's document links only go to a destination page, but PDF supports going to specific locations within that page. Proposed is the addition of an optional argument to include a target point if desired.

[StLange]

I added your code to PDFsharp and put it to our feature todo list for MigraDoc.