[docs] Internal classes API (huggingface#5513)

* internal classes api

* add internal class overview

* fix toctree

Author: stevhliu

docs/source/en/_toctree.yml

@@ -164,24 +164,14 @@
   title: Conceptual Guides
 - sections:
   - sections:
-    - local: api/activations
-      title: Custom activation functions
-    - local: api/normalization
-      title: Custom normalization layers
-    - local: api/attnprocessor
-      title: Attention Processor
-    - local: api/logging
-      title: Logging
     - local: api/configuration
       title: Configuration
-    - local: api/outputs
-      title: Outputs
     - local: api/loaders
       title: Loaders
-    - local: api/utilities
-      title: Utilities
-    - local: api/image_processor
-      title: VAE Image Processor
+    - local: api/logging
+      title: Logging
+    - local: api/outputs
+      title: Outputs
     title: Main Classes
   - sections:
     - local: api/models/overview
@@ -389,4 +379,18 @@
     - local: api/schedulers/vq_diffusion
       title: VQDiffusionScheduler
     title: Schedulers
+  - sections:
+    - local: api/internal_classes_overview
+      title: Overview
+    - local: api/attnprocessor
+      title: Attention Processor
+    - local: api/activations
+      title: Custom activation functions
+    - local: api/normalization
+      title: Custom normalization layers
+    - local: api/utilities
+      title: Utilities
+    - local: api/image_processor
+      title: VAE Image Processor
+    title: Internal classes   
   title: API

docs/source/en/api/internal_classes_overview.md

@@ -0,0 +1,3 @@
+# Overview
+
+The APIs in this section are more experimental and prone to breaking changes. Most of them are used internally for development, but they may also be useful to you if you're interested in building a diffusion model with some custom parts or if you're interested in some of our helper utilities for working with 🤗 Diffusers.