Add draft for guidance on how to manage Slack channels for briefs

[romaricpascal]

Starter for 10 on a guide to manage Slack channels during briefs. Up for any additions and content edits.

It includes information about:

• creating channels for each brief
• using tabs to store links to informations about the brief
• archiving channels after a brief (we don't really do that, but that would keep things tidy)

I think it could probably have an extra section about:

• preferring open communications in your squad's channel to DMs, as it allows other people to respond
• communication not being restricted to your squad's channel: if you have a question/notification for the rest of the team or need some help because you're missing this or that expertise, posting on the team channel (again, preferably to DMs) is a-ok.

[netlify]

✅ Deploy Preview for govuk-design-system-team-docs ready!

Name	Link
🔨 Latest commit	9e8d5c1
🔍 Latest deploy log	https://app.netlify.com/sites/govuk-design-system-team-docs/deploys/67a4dfb2d0675b0008a44f65
😎 Deploy Preview	https://deploy-preview-121--govuk-design-system-team-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.