Add pre-commit hook for django-upgrade

[SukiCZ]

I believe this repo is widely used as a hands-on experience with documentation, hence the repo should follow best Django (and Python) practicies.

One way to up 🆙 the best practicies is to follow django-upgrade (and pyupgrade) pre-commit hook. Pyupgrade is managed by Ruff PU rules

Using django-upgrade on current codebase would result in only changing request.META["HTTP_REFERER"] to request.headers["referer"] as defined by a Request Headers fixer. The rest of the codebase is awesome! 💥

Cheers 🍻 from 🇨‍🇿