Update docs for adding a new app
Update docs for adding a new app

- Use single quotes to wrap a string with no interpolation
- Add a hyphen/underscore to the app name to indicate that Puppet
  classes must contain underscores but DNS names must include hyphens
- Use calculators rather than courts_api as an example because courts_api
  has been removed
- Format the example Puppet code to look more like how we want it
- Change 'Nagios' to generic 'monitoring'
- Fix hieradata paths
- Remove references to preview
- Remove instructions for redeploying the current version of Puppet
  (this is no longer accurate)

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