Also had to update the test fixtures because mkdocs, bootstrap, whatever it may be have changed, so the sites end up slightly different.
Reviewed-on: #24
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>
The mechanism here is to shadow a specified `custom.theme_dir` in the
generated docs directory, and then ignore generated files therein (since they
will be being used by the theme to render the pages, and are not actually
part of the documentation).
