[Docs] Add documentation about social accounts

docs/admin/03_socialaccounts.rst

+Social accounts
+===============
+
+AlekSIS can authenticate users against third party applications using OAuth2
+or OpenID
+
+
+.. warning::
+  Socialaccounts are NOT working with two factor authentication! If a user
+  authenticates with a social account, the two factor authentication is
+  ignored! Will be fixed in a future release.
+
+Configuring social account provider
+-----------------------------------
+
+For available providers, see documentation of `django-allauth
+<https://django-allauth.readthedocs.io/en/latest/providers.html>`_.
+
+A new social account provider can be configured in your configuration file
+(located in ``/etc/aleksis/``).
+
+Configuration example::
+
+  [auth.providers.gitlab]
+  GITLAB_URL = "https://gitlab.exmaple.com"
+
+After configuring a new auth provider, you have to restart your AlekSIS
+instance and configure client id and secret in the Backend Admin interface,
+located at `https://aleksis.example.com/admin <https://aleksis.example.com/admin>`_.
+Click "Social applications" and add a new application. Choose your
+provider and enter client id and secret from your application and choose
+your site:
+
+.. image:: ../_static/create_social_application.png
+  :width: 400
+  :alt: Create social application