docs: Document GitLab custom CI/CD variables

We introduced the QEMU_CI_AVOCADO_TESTING variable in commit f56bf4ca
("gitlab: Run Avocado tests manually (except mainstream CI)"), but
forgot to document it properly. Do it now.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210727142431.1672530-2-philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>