ci: Use "when: always" for pages job

ci: Use "when: always" for pages job

"when: on_success" meant that that the job wouldn't run automatically
until all jobs of all earlier stages passed, and would be skipped if
any of them failed. But we need to always run this job if any
documentation files were modified.

Fixes: 8e2cb8ef276b "gitlab-ci: Extend .ci-run-policy template for docs
                     jobs"

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5711>