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>
3 jobs for master in 3 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Containers
passed #275737
container-build

00:02:50

 
  Build
passed #275738
check

00:00:36

 
  Deploy
manual #275739
allowed to fail manual
pages