Setup for Italian translation and add quicktour.mdx translation
I created the folder for files translated into Italian and made the translation of quicktour.mdx and index.mdx files.

The folder (called 'it') is located in transformers/docs/source/.
The translated documents are located in the transformers/docs/source/it folder.

Fixes [#17459](https://github.com/huggingface/transformers/issues/17459)


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?

@omarespejel @sgugger

_The documentation is not available anymore as the PR was closed or merged._<|||||>This looks great! Thanks @mfumanelli for the PR and opening the venue for an Italian 🇮🇹 translation!

Since this is the first IT translation could please add `it` to the languages section in the `.github/workflows` ([here](https://github.com/huggingface/transformers/blob/main/.github/workflows/build_documentation.yml) and [here](https://github.com/huggingface/transformers/blob/main/.github/workflows/build_pr_documentation.yml))?<|||||>Thank you @mfumanelli 🇮🇹! LGTM, @sgugger :)