Training tutorial (#2473)

* first draft

* ✨ minor edits

* ✨ minor fixes

* 🖍 apply feedbacks

* 🖍 apply feedback and minor edits

Author: stevhliu

docs/source/en/_toctree.yml

@@ -8,6 +8,10 @@
   - local: installation
     title: Installation
   title: Get started
+- sections:
+  - local: tutorials/basic_training
+    title: Train a diffusion model
+  title: Tutorials
 - sections:
   - sections:
     - local: using-diffusers/loading