gitlab-ci: Make URL checking run in paralle with everything else

There's little point in gating all the other jobs on a relatively slow
job as the URL checker, so make it run in parallel with everything else
without blocking anything.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -5,10 +5,11 @@ stages:
   - test
   - generate-html
   - generate-pdf
+  - test-urls
 
 test-urls:
   image: debian:buster-slim
-  stage: test
+  stage: test-urls
   tags:
     - lightweight
   before_script:
@@ -22,6 +23,7 @@ test-urls:
   # wouldn't be fair to block a merge due to an unrelated URL suddenly failing.
   # Treat this as a warning that the site has degraded.
   allow_failure: true
+  needs: []
 
 test-html:
   image: