Add explanation for the supported file types

Author: PProfizi

src/ansys/dpf/core/documentation/operator_doc_template.j2

@@ -13,6 +13,8 @@ license: {{ scripting_info.license }}
 {{ operator_description }}
 {% if is_router %}
 ## Supported file types
+
+This operator supports the following keys ([file formats](../../index.md#overview-of-dpf)) for each listed namespace (plugin/solver):
 {% for namespace, supported_files in supported_file_types.items() | sort %}
 - {{ namespace }}: {{ supported_files }} {% endfor %}
 {% endif %}