Change accessibility testing to `pa11y` from `a11y`

[apburnes]

In order to address dependabot security issues with the archived (a11y)[https://github.com/addyosmani/a11y] development dependency, switch accessibility testing to the (pa11y)[https://github.com/pa11y/pa11y].

Acceptance Criteria

• [ ] Remove a11y
• [ ] Add pa11y
• [ ] Update the npm test script to use pa11y command
• [ ] Add documentation on how to use the updated accessibility tests

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Security considerations

• Fixes dependabot alert on the archived a11y