Add guidance on graded browser support

[claireashworth]

What

Add a new page on how the team will provide graded browser support.

Why

As part of v5 the team is stopping or reducing support for older browsers.

Who needs to know about this

Tech writer, developers

Done when

• new page is drafted, reviewed and published
• new page has links between other related guidance

[claireashworth]

Link to draft document

[claireashworth]

Re-shared document with team for review

[claireashworth]

Team has reviewed. There is one change left for approval about Safari versions supported in which assigned browser grade. There are no 2 docs. a shorter doc for the frontend site and a more detailed doc to go onto GitHub.
Another tech writer or content designer should give it a quick review as well before we add it to the final release. Review not needed for the pre-release as we might get comments back and need to make changes.

[claireashworth]

Added Ollie and Romaric for final review on:

Browser support - short version
Browser support (full version)

Publishing the short version. Page list as follows:
Get started
NEW PAGE - Stay up to date with the latest Frontend changes (published)
NEW PAGE - Changes to GOV.UK Frontend v5.0.0 (#315)
NEW PAGE - How we provide support for different browsers (short version)

Publishing the full version:
Add to repo - docs/contributing/browser-support.md

[romaricpascal]

Closed by #356 and alphagov/govuk-frontend#4381