Rewrite / extend doc to rename specialist country
Rewrite / extend doc to rename specialist country

Previously the Specialist Publisher section of the manual had quite
a lot of steps. This combines some of the associated steps together,
and adds a final step to fix any affected email subscriber lists.

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