doc: Add 'administration_password' to index.

wiki/src/doc/first_steps.index.mdwn

@@ -8,6 +8,7 @@
       for [[Windows|first_steps/resetting_a_usb_stick/windows]]
   - [[Start Tails|first_steps/start_tails]]
   - [[Startup Options|first_steps/startup_options]]
+    - [[Administration Password|first_steps/startup_options/administration_password]]
     - [[Tor Bridge Mode|first_steps/startup_options/bridge_mode]]
     - [[Windows Camouflage|first_steps/startup_options/windows_camouflage]]
   - [[Introduction to GNOME and the Tails desktop|first_steps/introduction_to_gnome_and_the_tails_desktop]]