dev: add a11y tests on our docs site

[daine]

📇 User story

As a gcds developer, I want accessibility tests to run on our documentation website so that I can be sure it meets accessibility checks every time I update the site with a PR

✅ Definition of Done / Outcomes

• Done when there is a github action that runs an accessibility check
• Done when the github action runs on a PR
• Done when the github action runs on a deployment / when the website is published

📜 Acceptance criteria

• Test script runs for every page, both in EN and FR, in the documentation website
• Test script checks for dead links

📝 More info

Notify has these on their CI (from Andrew L):
Here is our workflow that tests staging every 3 hours for a11y:
https://github.com/cds-snc/notification-admin/blob/main/.github/workflows/cypress-staging.yaml
We also have a full suite of tests that run on every PR (including the same a11y checks as above):
https://github.com/cds-snc/notification-admin/blob/main/.github/workflows/test-admin-deploy.yaml#L153
We wrote a little helper to wrap axe and the html validator (and a few other things that were specific to problems we encountered, like dead links) here:
https://github.com/cds-snc/notification-admin/blob/main/tests_cypress/cypress/support/commands.js#L11

Using that helper, you could scan your component pages (or wherever) like this:

const component_pages = [
    'https://design-system.alpha.canada.ca/en/components/breadcrumbs/',
    'https://design-system.alpha.canada.ca/en/components/button/',
    'https://design-system.alpha.canada.ca/en/components/card/',
    'https://design-system.alpha.canada.ca/en/components/checkbox/',
    'https://design-system.alpha.canada.ca/en/components/container/',
    'https://design-system.alpha.canada.ca/en/components/date-input/'
];

describe("Check GCDS for a11y", () => {
    component_pages.forEach((page) => {
        it(`A11Y check for ${page}`, () => {
            cy.a11yScan(page, { a11y: true, htmlValidate: true, deadLinks: false, mimeTypes: false, axeConfig: false });
        });
    });
});

. 🚫 Out of scope
_ If possible, give precisions on what is not part of this work_

[ethanWallace]

Created a separate pull request to handle the component previews.