Auth0 is missing from docs

[Aurora-at-Othersphere]

Expected behaviour

Documentation exists for Auth0 which is supported in the code as a backend: https://github.com/python-social-auth/social-core/blob/master/social_core/backends/auth0.py

Actual behaviour

Describe what happens instead and why is it an issue. No documentation exits for Auth0 Backend in the expected location (or any location): https://github.com/python-social-auth/social-docs/blob/master/docs/backends/index.rst

Flagging this as an issue for anyone looking for inspiration for documentation improvement pull requests.