Confirm we can use custom configuration

[grayside]

Problem

Received a report that the image doesn't accept pa11y config as expected, need to put together a live test to see what changes might need to be made, such as documentation updates.

Solution

This might be a good excuse to propose a container-based approach to github.com/phase2/particle's pa11y support: https://github.com/phase2/particle/blob/master/tools/tests/accessibility/pa11y.js

Once we can identify what it takes to use custom config, we update the documentation to match.