Setup for Italian translation and add quicktour.mdx translation (#17472)

* Setup for Italian translation and add first document

- Add 'it' folder for files translated into Italian
- Add _config.py and _toctree.yml files
- Add translation of quicktour.mdx

* Fix style issue of italian documentation files

* Add 'it' to the languages section in the .github/workflows

* Remove - installation from _toctree for Italian

* Translation for index file

- Add index to _toctree.yml
- Add translation of index.mdx

* Fix typo in docs/source/it/index.mdx

* Translate code comments in docs/source/it/_config.py

Co-authored-by: Martina Fumanelli <martinafumanelli@Martinas-MBP.homenet.telecomitalia.it>

.github/workflows/build_documentation.yml

@@ -15,6 +15,6 @@ jobs:
       commit_sha: ${{ github.sha }}
       package: transformers
       notebook_folder: transformers_doc
-      languages: en es pt
+      languages: en es it pt
     secrets:
       token: ${{ secrets.HUGGINGFACE_PUSH }}