DOC: Reorganize plotting.rst
With the recent additions of hexbin, area, table, and pie charts (I may be forgetting some), the visualization docs are getting a bit long. I'll reorganize things to hopefully be a bit clearer.

I'll also mention things like seaborn, ggplot, mpld3, vincent, bearcart, and maybe remove the note that we plan to add more integration with maptlotlib.
- [x] link to matplotlib functions
- [x] all calls using random should be seeded so people can reproduce
