DOC: add hvplot as option to plotting.backend
#### Location of the documentation

Couple of sections here
https://pandas.pydata.org/pandas-docs/stable/ecosystem.html#visualization
https://dev.pandas.io/docs/development/extending.html#plotting-backends

#### Documentation problem

Add information that [`hvplot`](https://hvplot.holoviz.org/) can be used as a backend
`pd.set_option("plotting.backend", "hvplot")`

#### Suggested fix for documentation

Not sure will this could go. Could add a hvplot section to https://github.com/pandas-dev/pandas/blob/master/doc/source/ecosystem.rst#visualization but perhaps want to make an explicit list of all the plotting backend options at https://dev.pandas.io/docs/development/extending.html#plotting-backends
