Visual testing

[Josh-Cena]

With a lot of CSS refactors / polishings under the way, we should find a way to ensure they don't regress. Testing CSS on Infima is much easier than Docusaurus, and also much more vital.

I have taken some inspiration from TypeDoc's visual regression CI. From my discussion with Gerrit, it's supposed to be fairly easy, without app installation / service subscription. The only problem when it's not connected to some external service is there's no way to easily retrieve the testing artifacts.

I wonder if we can:
(a) Connect to an AWS S3/equivalent so we can store the snapshots
(b) Use a GH comment action to post the artifact
(c) Install the reg-suit app which seems promising

We can do our tests exclusively on the /demo page, which should make things a lot easier.