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>

docs/source/it/_config.py

@@ -0,0 +1,15 @@
+# docstyle-ignore
+INSTALL_CONTENT = """
+# Installazione di Transformers
+! pip install transformers datasets
+# Per installare dalla fonte invece dell'ultima versione rilasciata, commenta il comando sopra e
+# rimuovi la modalità commento al comando seguente.
+# ! pip install git+https://github.com/huggingface/transformers.git
+"""
+
+notebook_first_cells = [{"type": "code", "content": INSTALL_CONTENT}]
+black_avoid_patterns = {
+    "{processor_class}": "FakeProcessorClass",
+    "{model_class}": "FakeModelClass",
+    "{object_class}": "FakeObjectClass",
+}