Improve Document creation page to allow an user to select if the new document type is available in DAM when creating an Asset

In order to see the new document type in the "Asset type" drop-down list on the new Asset creation page, the following contribution must be added:

<extension target="org.nuxeo.dam.DamService" point="allowedAssetTypes">
    <allowedAssetType name="CustomAssetDocType" />
  </extension>