Rewrite section about freezing deployments
Rewrite section about freezing deployments

This now guides the reader to use the new "Freeze deployments?" feature
in the Release app, and explains the effect it will have. Since checking
a deploy freeze is really part of the deployment pipeline, this also
moves it into its own step/section.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs