Resolve disparities between design system and frontend

[anandamaryon1]

Having compared the service manual's design system with the nhsuk-frontend/nhsuk-prototype-kit (#1399), we have found that there are a few differences:

• frontend includes hero banner component, which is not documented in design system
• frontend includes feature card component, which is not documented in design system
• example content used in frontend differ from design system

We should resolve these differences, so that the frontend, prototype kit and design system are all aligned. This will reduce the chance of misunderstandings and confusion by people using these resources.