Refactor model summary (#21408)

* first draft of model summary

* restructure docs

* finish first draft

* ✨minor reviews and edits

* apply feedbacks

* save important info, create new page for attention

* add attention doc to toctree

* ✨ few more minor fixes

docs/source/en/_toctree.yml

@@ -157,9 +157,11 @@
   - local: tasks_explained
     title: How 🤗 Transformers solve tasks
   - local: model_summary
-    title: Summary of the models
+    title: The Transformer model family
   - local: tokenizer_summary
     title: Summary of the tokenizers
+  - local: attention
+    title: Attention mechanisms
   - local: pad_truncation
     title: Padding and truncation
   - local: bertology