Tokenization tutorial (#5257)

* All done

* Link to the tutorial

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

* Add metnion of the return_xxx args
Co-authored-by: Thomas Wolf <thomwolf@users.noreply.github.com>

docs/source/index.rst

@@ -139,6 +139,7 @@ conversion utilities for the following models:
 
     task_summary
     model_summary
+    preprocessing
     serialization
     model_sharing
     multilingual

docs/source/quicktour.rst

@@ -204,7 +204,7 @@ padding token the model was pretrained with. The attention mask is also adapted
      'attention_mask': tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
                                [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0]])}
 
-You can learn more about tokenizers on their :doc:`doc page <main_classes/tokenizer>` (tutorial coming soon).
+You can learn more about tokenizers :doc:`here <preprocessing>`.
 
 Using the model
 ^^^^^^^^^^^^^^^