Add release notes for GOV.UK Prototype Kit v9.13.0

[EoinShaughnessy]

This PR is to tell users:

• they can opt in to link styles
• about fixes to make Sass errors clearer and allow Node 15 to be used

[EoinShaughnessy]

Need to add link to pull request.

[EoinShaughnessy]

@36degrees Thanks for reviewing! I've actioned your feedback (or tried to).

Just wondering about a few final bits before we send this content for pre-i. In particular:

• Are the features listed in the right order? (I've done this by PR number, but what would users want to see first?)
• How's the Sass feature description?
• Is it weird to give the Frontend update its own heading? (As you say, previously, we've just bulleted such updates - but doing that here might look 'off,' because the other features have descriptions.)

[36degrees]

Are the features listed in the right order? (I've done this by PR number, but what would users want to see first?)

Looks good to me 👍🏻

How's the Sass feature description?

Generally looks great, but I have a couple of suggestions for the first sentence just to make it slightly clearer.

Is it weird to give the Frontend update its own heading? (As you say, previously, we've just bulleted such updates - but doing that here might look 'off,' because the other features have descriptions.)

No, I think that works well 👍🏻

[EoinShaughnessy]

Release notes have now passed pre-i, so I've sent them for 2i.

[EoinShaughnessy]

@36degrees Before we move the linked issue to Ready to Release/Done, do we need to add a link to the (as-yet-uncreated) PR for the Frontend release?

[36degrees]

We won't know what the PR is until we've released GOV.UK Frontend, so we'll have to make that change before we merge this. Other than adding that, is this ready to go?

[EoinShaughnessy]

@36degrees Yeah, it's ready to go.