add doc (developer guide)

Author: yiyixuxu

docs/source/en/_toctree.yml

@@ -86,6 +86,9 @@
   - local: hybrid_inference/api_reference
     title: API Reference
   title: Hybrid Inference
+- sections:
+  - local: modular_diffusers/developer_guide
+    title: Developer Guide
 - sections:
   - local: using-diffusers/cogvideox
     title: CogVideoX