Implement Django bindings

[rafalp]

We've originally planned to have separate django_ariadne repo to keep bindings for django, but we've discussed this internally and decided to give ariadne.contrib.django a try instead. This has advantage of keeping all features in single repo, but it may be tricker to test if we add more bindings for other projects to the repo.

[rafalp]

We now have GraphQL boilerplate that makes it super easy to add API to Django, and we also have a working example in our docs, so closing this for now.

We may follow up with separate repo in the future as we find the need for such.