Quick tour (#5145)

* Quicktour part 1

* Update

* All done

* Typos

Co-authored-by: Thomas Wolf <thomwolf@users.noreply.github.com>

* Address comments in quick tour

* Update docs/source/quicktour.rst

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update from feedback

Co-authored-by: Thomas Wolf <thomwolf@users.noreply.github.com>
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

docs/source/index.rst

@@ -38,6 +38,16 @@ Choose the right framework for every part of a model's lifetime:
 Contents
 ---------------------------------
 
+The documentation is organized in five parts:
+
+- **GET STARTED** contains a quick tour, the installation instructions and some useful information about our philosophy
+  and a glossary.
+- **USING TRANSFORMERS** contains general tutorials on how to use the library.
+- **ADVANCED GUIDES** contains more advanced guides that are more specific to a given script or part of the library.
+- **RESEARCH** focuses on tutorials that have less to do with how to use the library but more about general resarch in
+  transformers model
+- **PACKAGE REFERENCE** contains the documentation of each public class and function.
+
 The library currently contains PyTorch and Tensorflow implementations, pre-trained model weights, usage scripts and
 conversion utilities for the following models:
 
@@ -118,16 +128,17 @@ conversion utilities for the following models:
     :maxdepth: 2
     :caption: Get started
 
+    quicktour
     installation
-    quickstart
+    philosophy
     glossary
 
 .. toctree::
     :maxdepth: 2
     :caption: Using Transformers
 
-    usage
-    summary
+    task_summary
+    model_summary
     serialization
     model_sharing
     multilingual