A message like It can only be used for development and will be stopped if used in production means that you are using Nuxeo-provided prepackaged hotfixes, which are a paid service. You must uninstall the prepackaged hotfixes if you don't have a support contract and therefore a valid license (instance.clid) file.

Otherwise you can always compile everything from source. But not use our prepackaged hotfixes directly.

The full source of Nuxeo is available at https://github.com/nuxeo, under an open source license (see https://doc.nuxeo.com/nxdoc/licenses).

you can create a doctype with simple XML contributions using org.nuxeo.ecm.core.schema.TypeService extension points (schema and doctype)