Flask-digestauth

* Changed the package name from flask-digest-auth to Flask-DigestAuth, according to the Flask recommended extension guidelines https://flask.palletsprojects.com/en/latest/extensiondev/ .
* Replaced app.digest_auth with app.extensions["digest-auth"] to store the DigestAuth instance.
* Replaced auth.app with current_app, to prevent circular imports.

Fixed the missing authentication state with disabled users.

Changed the visibility of several methods and properties of the DigestAuth class that should be private to private.

Fixed the pytest example in the documentation.

Fixed the dependencies for the documentation hosted on Read the Docs.

Added the Sphinx documentation, and hosted the documentation on Read the Docs.