[DOCS]: Use Virtual WebGL on all docs pages (or at least the ones that need it)

[emilykl]

Docs pages with many WebGL plots often don't load correctly due to too many WebGL contexts:

Example: https://plotly.com/python/splom/

or

It's especially bad on that page because the first plot is usually the one which doesn't load.

The simplest change would be to add Virtual WebGL to the docs template and load it on every page.

cc @alexcjohnson