Suppress matplotlib plots on docs build

[bmorris3]

This is a version of the improvement in PR astropy/package-template#453. Users attempting to build docs before this PR will find that the matplotlib plot directive renders plots in individual, interactive windows. After this PR, the figures are rendered in the agg backend and not in the display.

[Cadair]

Thanks for the PR! This is also useful if you start testing figures i.e. with pytest-mpl

Would you be willing to add a section on environment variables / extra config in tox.ini with this as an example to the tox docs page? I am trying to keep this repo as documentation first and not have (too much) stuff buried in the template without mention in the narrative documentation.

[bmorris3]

@Cadair Sure thing! Would it be useful to include a small section about adding sphinx extensions? Happy to write a paragraph about mpl plot directive, for example.

[bmorris3]

I went ahead and added a bit of text for using the plot directive in 7022fcf.