Document how to use multi-theme tests to preview docs across themes

[choldgraf]

In sphinx-panels @chrisjsewell has worked out a cool way to test documentation against multiple themes, for projects that need to work across many of these themes. We should document this in a general place so that it's easy for folks to copy the pattern and apply to new repositories.

@chrisjsewell mentioned a few relevant links:

see https://github.com/executablebooks/sphinx-panels/blob/master/tox.ini and https://github.com/executablebooks/sphinx-panels/blob/d25990bece19c33bb07975048f4843025fb179af/docs/conf.py#L68

setting it up with the HTML_THEME environmental variable means the only difference in the branches is just the default theme: https://github.com/executablebooks/sphinx-panels/compare/alabaster-theme, which is pretty general