[Modular]add a real quick start guide (huggingface#13029)

* add a real quick start guide

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

* update a bit more

* fix

* Apply suggestions from code review

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

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

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

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

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

* update more

* Apply suggestions from code review

Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

* address more feedbacks: move components amnager earlier, explain blocks vs sub-blocks etc

* more

* remove the link to mellon guide, not exist in this PR yet

---------

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

Author: 3 people

docs/source/en/modular_diffusers/overview.md

@@ -24,7 +24,7 @@ The Modular Diffusers docs are organized as shown below.
 
 ## Quickstart
 
-- A [quickstart](./quickstart) demonstrating how to implement an example workflow with Modular Diffusers.
+- The [quickstart](./quickstart) shows you how to run a modular pipeline, understand its structure, and customize it by modifying the blocks that compose it.
 
 ## ModularPipelineBlocks