Add Italian translation for `add_new_model.mdx` (#18713)

* fix conflicts

* start translating

* proof check

* add toc

* fix errors and typos

docs/source/it/_toctree.yml

@@ -39,3 +39,7 @@
 - sections:
   - local: add_new_pipeline
     title: Come aggiungere una pipeline a 🤗 Transformers?
+  - local: add_new_model
+    title: Come aggiungere un modello a 🤗 Transformers?
+  title: Guide How-to
+