Add Auth0 setup documentation

There is currently no documentation on how to set up Auth0 for an individual deployment of Hackplatform. The following steps should be documented:

1. Registration to Auth0
2. Setting up a frontend and a backend app (client secrets, etc.)
3. Setting up the Authorization extension
3.1. Provide example data to Import into the plugin (Roles, Permissions)
3.2. Provide example Rule to copy over data to idToken
3.2 Document additional setup (APIs?)