doc(grdf): add bo fetching access token

docs/ecolyo-agent/Technical/grdf.md

+# GRDF Access Token
+
+To avoid the creation of a new access token for each connectors, the backoffice is responsible for fetching access tokens.
+
+It fetches a new access token every 2 hours and stores it in a table `grdf_access_tokens`.
+
+Each connectors will fetch an endpoint to get the access token instead.