Write some draft messaging to communicate v4 and how it's different from v5

[CharlotteDowns]

What

Draft some messaging that can be adapted across channels (incl. Readme.md, V4.10 release notes, Changelog and slack support channels) to help users find information specific to v4.

Why

We asked v4 users as part of 'Investigate what things v4 users need to confirm their service aligns with the brand updates #5893' how they currently approach updating their dependencies and where they look for information.

A large majority of the users we spoke to are blocked to make major updates for varying factors and tend to go to the latest guidance/documentation first.

If we can help users within our existing communications methods it could be beneficial.

Done when

• All messaging is outlined in a Google document
• reviewed by the team
• posted and/or added to the channels identified in the 'What'

[CharlotteDowns]

Google document for v4 messages across channels.

[CharlotteDowns]

Linked pull requests

• Design System website guidance: Pull request to add the content to website including the What's new section
• Frontend tech docs: Pull request to add brand content to Changes to GOV.UK Frontend v5.0.0 page

[owenatgov]

Moving this into blocked because the content structure relies in part on #5892 being reviewed