doc: Fix reference to xworkflows doc inventory.

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>
add a DjangoImplementationWrapper (Closes #10).

* Used by django_xworkflows.Workflow
* Sets ``alters_data`` and ``do_not_call_in_templates`` for proper
  template handling.
Document and cleanup after PR #10.

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>