Update to the Zendesk doc
Update to the Zendesk doc

There is a best practices slidedeck which developers might find useful
as an introduction. I've also moved the advice for DGU tickets into
it's own section so that it is easier to find.

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