Spike: establish a baseline using Chromatic's accessibility regression testing

This feature has been unlocked, now we have upgraded to Storybook 9

https://www.chromatic.com/docs/accessibility/#in-ci-regression-checks-with-chromatic

Accessibility regression testing using Chromatic, Storybook and axe provides an accessibility baseline by running a suite of tests against our components defined in Storybook.

We should try turning on an CI regression checks to see what our baseline looks like.

- If it's good, maybe we can run it all the time (note: there will be a cost for doing this, each accessibility snapshot counts as 0.x chargeable snapshots)
- If it's not so good, we should think further about how we might tackle the highest priority findings