Add user docs about 2fa

docs/user/01_personal_account.rst

@@ -21,7 +21,26 @@ preferences (cf. [@sec:PersonalPreferences]).
 Setup two-factor authentication
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-TBA.
+
+.. image:: ../_static/2fa.png
+  :width: 600
+  :alt: Configure two factor authentication
+
+AlekSIS provides two factor authentication using hardware tokens such as
+yubikeys which can generate OTPs or OTP application.
+
+To configure the second factor, visit `Account → 2FA` and follow the
+instructions.
+
+Please keep the backup codes somewhere safe so you do not lose access to
+your account. If you are unable to login with two factor authentication,
+please contact your site administrator.
+
+If you forget to safe your backup codes, but you are still logged in, visit
+`Account → 2FA`, and press `Show codes`.
+
+To disable two factor authentication, login to your account an navigate to
+`Account → 2FA`, then press the big red button to disable 2fa.
 
 Change password
 ~~~~~~~~~~~~~~~