Add rich table support for PDF backend

[jspast]

Now that rich tables are supported by the HTML (#2324) and MSWord (#2291) backends, the PDF backend could also significantly benefit from that.

I think this is being worked on internally, but I decided to create this feature request as there is no open PR. From what I can see, this would require a major refactor of the entire backend. I would like to help though honestly am not sure where to start.

Related to #1613