adds hedgedoc db secrets

backend/areas/apps/templates/stackspin-hedgedoc-variables.yaml.jinja

+apiVersion: v1
+kind: Secret
+metadata:
+  name: stackspin-hedgedoc-variables
+data:
+  hedgedoc_mariadb_password: "{{ 32 | generate_password | b64encode }}"
+  hedgedoc_mariadb_root_password: "{{ 32 | generate_password | b64encode }}"