Move support branches docs to team docs

The process for using a support branch is being moved into our team docs as part of alphagov/design-system-team-docs#21.

Remove the existing documentation and add a reference to it in our docs on publishing releases. Update those docs to reflect the fact that this branch is a support branch.

Author: 36degrees

docs/releasing/publishing-from-a-support-branch.md

@@ -1,6 +1,10 @@
 # Before you publish GOV.UK Frontend
 
-Read the docs for [what to do before publishing a release](/docs/releasing/before-publishing-a-release.md) to ensure you are prepared to publish.
+This is a support branch, which we use to release additional v4.x versions now that the `main` branch reflects a newer major version.
+
+Make sure you understand how this branch is meant to be used be reading the [documentation on support branches](https://govuk-design-system-team-docs.netlify.app/how-we-work/version-control/support-branches.html#support-branches) in our team docs.
+
+You should also read the docs for [what to do before publishing a release](/docs/releasing/before-publishing-a-release.md) to ensure you are prepared to publish.
 
 # Publish a new version of GOV.UK Frontend