Update test app

[GCHQ-Developer-530]

Update test app to have more pages including more components to test in Cypress and React Testing Library.

Tidy up code base, add contribution guide and license.

Ticket: #8 and https://github.com/mi6/ic-ui-kit/issues/1971

Further improvements suggested in the following tickets:

• Deploy the app
• Improve PR check workflow
• Improve commit process
• Improve testing

This PR should just be an extension and tidy up that should make it possible to make the repo public and unblock https://github.com/mi6/ic-design-system/issues/1008

[GCHQ-Developer-465]

Have given feedback internally 😊