doc: Add 'trusting_tails_signing_key' the documentation index.

wiki/src/.htaccess

@@ -20,3 +20,4 @@ RewriteRule ^doc/installing_onto_a_usb_stick/windows doc/first_steps/manual_usb_
 RewriteRule ^doc/installing_onto_a_usb_stick/mac doc/first_steps/manual_usb_installation/mac
 RewriteRule ^doc/first_steps/bridge_mode doc/first_steps/startup_options/bridge_mode
 RewriteRule ^doc/first_steps/windows_theme doc/first_steps/startup_options/windows_camouflage
+RewriteRule ^doc/trusting_tails_signing_key doc/get/trusting_tails_signing_key

wiki/src/doc/get.index.mdwn

@@ -4,6 +4,7 @@
     - [[Using Gnome: Ubuntu, Debian, Tails, etc.|get/verify_the_iso_image_using_gnome]]
     - [[Using Linux with the command line|get/verify_the_iso_image_using_the_command_line]]
     - [[Using other operating systems|get/verify_the_iso_image_using_other_operating_systems]]
+    - [[Trusting Tails signing key|get/trusting_tails_signing_key]]
   - [[Burn a DVD or install onto a USB stick|download#media]]
   - [[Stay tuned|download#stay_tuned]]
   - [[Start Tails!|download#start]]

wiki/src/download.html

@@ -178,7 +178,7 @@ share it without restriction.</strong>
 	Tails signing key you downloaded:</p>
 
 	<ul>
-	  <li>[[Trusting Tails signing key|doc/trusting_tails_signing_key]]</li>
+	  <li>[[Trusting Tails signing key|doc/get/trusting_tails_signing_key]]</li>
 	</ul>
 
 	<!-- <h2>FIXME: What to do if the image is bad?</h2> -->