DOC: Reorganize visualization.rst

- add matplotlib to intersphinx
- put random seeds in supress and note
- use versionadded in more places
- link to api on more methods
- title case titles
- reorganize hierarchy

The new hierarchy is

- Basic Plotting (line plots on Frame and Series)
- Other Plots (bar, scatter, etc.)
- Plot formatting

DOC: add ecosystem note to top of visualization.rst

DOC: use cubehelix, not jet

fix line lengths on headers

reorg hierarchy to basic, other, and styling

Change matplotlib url
Merge pull request #7027 from TomAugspurger/visualization-rewrite

DOC: Visualization reorganization
ENH: cythonize groupby count
VB: add a vbench for groupby count
BUG: numeric_only must be False for count
CLN: EAFP for count