Update Django documentation

[Frzk]

It seems we still advise to use dj-static in the "Serving Static Files" section (https://doc.scalingo.com/languages/python/django/start#static-file-serving)
dj-static is deprecated, so we should update this section and remove it.

[ksol]

Indeed. A customer just told me that "whitenoise" is the preferred option, and it works with wsgi and asgi apps

[Frzk]

We also had customer that made a PR with dj-static, which has been merged.
Maybe it's still popular in the ecosystem?

Consequently, I suggest to:

• Switch to whitenoise, since it seems to be the state-of-the-art nowadays.
• Also keep dj-static, with a warning (and maybe explaining how to switch from dj-static to whitenoise.
• Add a section about Static: https://pypi.org/project/static/

[ksol]

I'm not familiar at all with the state of the python/django ecosystem, but dj-static has been archived since 2021, so personally I would have removed anything related to it except a disclaimer/warning specifying that whitenoise is the de-facto option and dj-static should not be considered.

Then again, that's not a really informed stance