[docs] Refactor image quality docs (huggingface#7758)

* refactor

* code snippets

* fix path

* fix path in guide

* code outputs

* align toctree title

* title

* fix title

Author: stevhliu

docs/source/en/_toctree.yml

@@ -63,12 +63,10 @@
     title: Pipeline callbacks
   - local: using-diffusers/reusing_seeds
     title: Reproducible pipelines
-  - local: using-diffusers/control_brightness
-    title: Control image brightness
+  - local: using-diffusers/image_quality
+    title: Controlling image quality
   - local: using-diffusers/weighted_prompts
     title: Prompt techniques
-  - local: using-diffusers/freeu
-    title: Improve generation quality with FreeU
   title: Inference techniques
 - sections:
   - local: using-diffusers/sdxl

docs/source/en/api/pipelines/overview.md

@@ -97,6 +97,11 @@ The table below lists all the pipelines currently available in 🤗 Diffusers an
 	- to
 	- components
 
+
+[[autodoc]] pipelines.StableDiffusionMixin.enable_freeu
+
+[[autodoc]] pipelines.StableDiffusionMixin.disable_freeu
+
 ## FlaxDiffusionPipeline
 
 [[autodoc]] pipelines.pipeline_flax_utils.FlaxDiffusionPipeline