[Docs] Add documentation about mailing

docs/admin/18_mail.rst

+Mail
+====
+
+AlekSIS needs to send mails e.g. for account confirmations, feedback or
+error reports.
+
+Configure mailing
+-----------------
+
+The mailserver can be configured via the configuration file
+
+.. code-block:: toml
+
+	[mail.server]
+	host = "mail.example.com"
+	tls = False
+	ssl = True
+	port = 25
+	user = "mailuser"
+	password = "password"
+
+Name and address for mails sent by AlekSIS can be configured in the
+webinterface.  To configure, visit `Admin → Configuration` and click on the
+`Mail` tab.
+
+Configure mail recipients
+-------------------------
+
+You can configure admin contacts in your configuration file, located at
+``/etc/aleksis/``.
+
+.. code-blocK:: toml
+
+	[contact]
+	admins = [["AlekSIS - Admins", "root@example.com"],["AlekSIS - Admins2", "root2@example.com"]]
+	from = 'aleksis@example.com'

docs/admin/22_registration.rst

 Registration and user invitations
 =================================
 
+User invitations and registration can be used to allow e.g. guardians, that
+don't have an account managed by the school, to use AlekSIS.
+
+
 Configuration
 -------------
 
@@ -22,11 +26,11 @@ User invitations
 User invitation can be enabled and configured via the configuration
 interface in frontend.
 
-In the ``Authentication`` tab, click the checkbox ``Enable invitations``, to
-enable invitations.
+* Authentication
 
-You can also configure the length of the invite code, defaults to ``3``, and
-the size of packets, defaults to ``5``.
+  * Enable invitations: Click to enable invitations.
+  * Length of invite code: Length of invitation code packets, defaults to 5.
+  * Size of packets: Configure how many packets are generated, defaults to 3.
 
 By default, an invitation code looks like the following:
 ``abcde-abcde-abcde``

docs/user/01_registration.rst

 Register a new account
-====================
+======================
 
 .. _core-user-registration: