[Modular] Add Custom Blocks guide to doc (#12339)

* update

* update

* Update docs/source/en/modular_diffusers/custom_blocks.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Update docs/source/en/modular_diffusers/custom_blocks.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Update docs/source/en/_toctree.yml

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Update docs/source/en/modular_diffusers/custom_blocks.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Apply suggestion from @stevhliu

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Apply suggestion from @stevhliu

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* update

* update

* update

* Apply suggestion from @stevhliu

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* Apply suggestion from @stevhliu

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

* update

* update

* update

* update

* update

* Update docs/source/en/modular_diffusers/custom_blocks.md

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

---------

Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>

Author: DN6

docs/source/en/_toctree.yml

@@ -121,6 +121,8 @@
     title: ComponentsManager
   - local: modular_diffusers/guiders
     title: Guiders
+  - local: modular_diffusers/custom_blocks
+    title: Building Custom Blocks
   title: Modular Diffusers
 - isExpanded: false
   sections: