GitLab CI: Try to keep the Documentation job from blocking

As https://docs.gitlab.com/ee/ci/yaml/index.html#allow_failure says:

"The default value for allow_failure is:

- true for manual jobs.
- false for jobs that use when: manual inside rules.
- false in all other cases."

Set "allow_failure: true" for the Documentation job.

[skip ci]

.gitlab-ci.yml

@@ -503,6 +503,7 @@ Documentation:
         - "epan/wslua/**/*"
     - if: '$CI_PIPELINE_SOURCE == "push"'
       when: manual
+      allow_failure: true
   script:
     # XXX We might want to move this to wireshark-ubuntu-dev or debian-setup.sh.
     - DEBIAN_FRONTEND=noninteractive apt-get update