Add external resources in mkdocs yaml

mkdocs.yml

@@ -101,6 +101,7 @@ nav:
       - index.md: tutorials/index.md
       - API: tutorials/01_api/
       - OEP Client: tutorials/02_oep_client/
+      - External Resources: tutorials/03_external/
       - Other: tutorials/99_other/
   - Questions: questions.md
   - Metainfo: meta.md