docs(812): add accessibility statement to docs site

[JohnTParsons]

closes #812

What

1. Background - accessibility awareness
2. What did you do - added statement
3. What does the reviewers should expect - new footer link and page

I have done:

• Written unit tests against changes
• Written functional tests against the component and/or NewsKit site
• Updated relevant documentation

I have tested manually:

• The feature's functionality is working as expected on Chrome, Firefox, Safari and Edge
• The screen reader reads and flows through the elements as expected.
• There are no new errors in the browser console coming from this PR.
• When visual test is not added, it renders correctly on different browsers and mobile viewports (Safari, Firefox, small mobile viewport, tablet)
• The Playground feature is working as expected

Before:

After:

Who should review this PR:

How to test:

[pp-serviceaccount]

You can preview these changes on:
Documentation Site: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/812-a11y-statement
Storybook: http://ncu-newskit-docs-pr.s3-website-eu-west-1.amazonaws.com/812-a11y-statement/storybook

[jps]

Is this a good e2e test? I'm not so sure, I agree that these won't change that frequently and it's possibly not much of an issue but what feels off to me is that it's very tightly coupled to the content, conceptually to me, this is very close to a snapshot test 🤔

[JohnTParsons]

Change pushed to remove footer-navigation.cy.js

[mutebg]

IMO the only useful test in the header navigation is should contain burger menu when on mobile. the rest are kind of dom checkings that can and should be in jest

[JohnTParsons]

@mutebg As requested, new header cypress tests removed as well