Add Italian (it) locale to the website

### Summary

The Italian documentation already exists upstream in [`php/frankenphp/docs/it`](https://github.com/php/frankenphp/tree/main/docs/it), but the website does not build or serve an Italian version yet. Every other translated language (fr, es, ja, zh, ru, tr, pt-br) is wired into `hugo.yaml` and `clone-documentation.php` — Italian is simply missing.

### Motivation

This was requested by the community. Massimiliano Arione (freelance web developer & Symfony specialist) asked:

> Could we give a push to the Italian docs translation, which is in the repo but not on the website? Thanks

### Proposal

Enable the `it` locale end to end, mirroring the existing languages:

- Register `it` in `hugo.yaml` (menu, weight, `contentDir`)
- Add `it` to the `languages` array in `clone-documentation.php` so the Italian docs get cloned at build time
- Translate the site chrome: `i18n/it.yaml`, `data/it/**` (home + help pages), `content/it/help`
- Add the missing `static/img/flags/it.svg` for the language switcher

### Type

Enhancement (new feature) — it adds a new supported language to the site, rather than fixing broken behavior.

Compare an existing language in hugo.yaml, clone-documentation.php, i18n/, data/, content/, and static/img/flags/. Add the Italian counterparts named in the proposal and run the site build to verify the docs clone, language menu, homepage, help pages, and it.svg all work. Done means the website builds and serves the Italian documentation end to end.

Written by the indexing model from the issue text.