Get rebrand guidance ready to publish

[calvin-lau-sig7]

What

Finalise brand implementation content, updating as work progresses on other squads.

Working content doc - branding MVP

Pull request of rebrand content prototype:
#4547

Why

We want content to be finalised and ready alongside v5.10 release

Who needs to be involved

Content designer, interaction designer

Who needs to review

Developer, other squads

Done when

Component guidance

• Pair with Owen to finalise guidance on how to toggle the new brand

Feature flag guidance

• Draft mvp feature flag guidance for users who do not use Nunjucks macros

Review

• Add finalised content into PR
• Final review with team
• Ready to publish