DOC: Add datamodel.yml for query-template datamodel

tools/query_template_datamodel.yml

+# Insert this into your LinkAhead datamodel to enable query templates.
+UserTemplate:
+  obligatory_properties:
+    Query:
+      datatype: TEXT
+      description: Query string of the query template, see https://docs.indiscale.com/caosdb-webui/extension/query_templates.html
+    templateDescription:
+      datatype: TEXT
+      description: Description of the query template which will be readable to the user.