Move page about Puppet class assignment
Move page about Puppet class assignment

This currently lives in the developer docs. Because it only covers
puppet, it's better at home in this repo. This will also increase the
likelihood that it will be kept up to date.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet