Create an automated task to check the accessibility of the whole website

[owenatgov]

What

Add a job to our CI suite to expand our existing automatic accessibility checks to chedk the accessibility of the whole website. This doesn't need to be part of our regular automated tests but can be a manual test we run when we want or possibly a non-required test.

Why

This is enshrining work done in #2957 to apply the work and findings from that. Generally it would be useful to be able to run this either once in a specified timeframe or whenever we want to ensure we're not making accessibility mistakes.

We revisited this for https://github.com/alphagov/design-system-team-internal/issues/912 (internal repo) and found this work useful for specifically quickly checking heading hierarchy and fixing it.

Who needs to work on this

Developer

Who needs to review this

Design system devs

Done when

• Existing test is converted into separate job
• Job is published
• Job is tested after merging to ensure it works in our github CI