DOC: move trellis under visualization.rst
Closes https://github.com/pydata/pandas/issues/8351

Just moved it and changed a few heading levels. I could add a note about considering seaborn. I've never used rplot.

ok merge when ready
 @jreback The travis faliure was from the openpyxl stuff, which you fixed in https://github.com/pydata/pandas/commit/8069f4779cd1b0fe84abc53c73d8e6d0a44b0674

OK to merge still?
 yep go ahead 
 Thanks, I'll keep an eye on this build.
