gitlab-ci: Make pages job manual

To stop sending those pesky failure emails, that condition people to
ignore anything sent by gitlab-ci.

Now we will have to click the button manually, whenever we want to bump
the docs.

Bugzilla: freedesktop/freedesktop#46


Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

.gitlab-ci.yml

@@ -28,6 +28,7 @@ check:
       - make check
 
 pages:
+    when: manual
     stage: deploy
     script:
       - make html