ONNX documentation (#5992)

* Move torchscript and add ONNX documentation under modle_export

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Let's follow guidelines by the gurus: Renamed torchscript.rst to serialization.rst

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* Remove previously introduced tree element

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* WIP doc

Signed-off-by: Morgan Funtowicz <funtowiczmo@gmail.com>

* ONNX documentation

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Fix invalid link

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Improve spelling

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

* Final wording pass

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

docs/source/index.rst

@@ -157,8 +157,8 @@ conversion utilities for the following models:
     notebooks
     converting_tensorflow_models
     migration
-    torchscript
     contributing
+    serialization
 
 .. toctree::
     :maxdepth: 2