DOC: cleanup vis docs
DOC: Give KDE a section header

DOC: remove extra slice in hexbin example

DOC: move notes to look nicer

A few changes that came up in https://github.com/pydata/pandas/pull/7027

OK!
 :+1: 
 seems 'pie' is missing from the Other Plots ?

also maybe make it `bar` (with the link), or `barh` (with a link to the same place)
w/o the quotes? 

http://pandas-docs.github.io/pandas-docs-travis/visualization.html#other-plots
