Document new fixtures hidden property

[36degrees]

What

• Update the example in 'Test if your HTML matches GOV.UK Frontend' to include the hidden property
• Add hidden to the list of properties under the example, with an appropriate description

Why

In 'Test if your HTML matches GOV.UK Frontend', we provide a description of the fixtures found in a components fixtures.json.

alphagov/govuk-frontend#2031 adds the hidden property to the objects in the fixtures list, so we need to update the documentation appropriately.

Who needs to know about this

@EoinShaughnessy, @hannalaakso, @36degrees

Done when

• Example updated
• List of properties updated
• Changes 2i'ed