Add hide this page component to documentation

[owenatgov]

What

Add the Hide This Page component plus guidance to GOV.UK Design System website.

Part of https://github.com/alphagov/govuk-design-system/issues/1755

This is a draft pull request to get an early review of the current implementation. It is not the final proposal for this component. Prior research into this component and future accessibility testing is being collated and discussed and will be referenced here, along with a more detailed description of the component, once we are ready to publish.

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[36degrees]

Has this been superseded by #2721?

[owenatgov]

@36degrees I think it has! This was originally a re-implementation of the original draft guidance written by Tim to test out the revised component. #2721 is more up to date than this one so lets close it.