Add content purpose supergroup to finder schema
Add content purpose supergroup to finder schema

This lets us create finders using [content purpose supergroups](https://github.com/alphagov/govuk_document_types/blob/5d572a6439c20593d448c83cfc68e21e82d49b91/data/supertypes.yml#L486-L572)!

We need this in this instance so we can match up the latest content for orgs
page with what is in the atom feeds and in the "latest content from" section
on the org pages themselves.

alphagov/govuk-content-schemas,alphagov/govuk-content-schemas