gitlab: only let pages be published from default branch

gitlab: only let pages be published from default branch

GitLab will happily publish pages generated by the latest CI pipeline
from any branch:

https://docs.gitlab.com/ee/user/project/pages/introduction.html

  "Remember that GitLab Pages are by default branch/tag agnostic
   and their deployment relies solely on what you specify in
   .gitlab-ci.yml. You can limit the pages job with the only
   parameter, whenever a new commit is pushed to a branch used
   specifically for your pages."

The current "pages" job is not limited, so it is happily publishing
docs content from any branch/tag in qemu.git that gets pushed to.
This means we're potentially publishing from the "staging" branch
or worse from outdated "stable-NNN" branches

This change restricts it to only publish from the default branch
in the main repository. For contributor forks, however, we allow
it to publish from any branch, since users will have arbitrarily
named topic branches in flight at any time.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210723113051.2792799-1-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>