Add gallery styling example for each backend

## Request

Add gallery styling examples for each backend

## Motivation

It is easy and relatively obvious how to use hvPlot for data exploration and scientific reporting.

It is not very obvious how to transform hvPlot output into something branded for an organization and useful in documents, presentations and web pages. This should also be easy and obvious.

I believe a first attempt at solving this would be an end to example for each backend with additional reference links to further resources.