[SPIKE] Header and footer changes to the page template

[domoscargin]

What

Using the v5 support branch, spike decoupling the <header> tag from the GOV.UK Header component, and the <footer> tag from the GOV.UK Footer component.

Why

We want to decouple these things to enable teams to change the <header> and <footer> without having to reimplement the Header and Footer components. We want to:

• test how these changes affect the Design System website, the Prototype Kit and the review app
• decide whether we can/should implement this as a non-breaking change
• draft some guidance for the change

Who needs to work on this

Developer, Accessibility specialist

Who needs to review this

Developer, Accessibility specialist, Content Designer/Technical writer

Done when

• We have decoupled the <header> and <footer> from their respective components
• We have this working in preview on the DS website, the Prototype Kit and the review app
• We have drafted some guidance for the change
• We have decided whether we are going to implement the change as breaking or non-breaking