That [[ https://github.com/encode/django-rest-framework/blob/master/rest_framework/utils/encoders.py | encoder ]] is used when querying the API for "application/json" content type.
It handles more encoding cases than the default one (notably it knows how to
encode a `swh.model.collections.ImmutableDict`), so let's also use it when
generating HTML documentation for the API endpoints.