Only rebuild documentation pages when source has changed

We also need to investigate an easy way to allow the pages job to be run in a MR and for the target url to be different from the real live documentation site. For info, see the summary notes on 📌 Formatting fixes Active . Maybe we can utilize the alternative url setting for this?

I already tested a while back the possibility of testing the "pages" job with a MR-specific URL and it's not possible: 📌 Allow testing documentation pages via MRs Active

I know that it requires docker locally and that the URL cannot be shared, but this reproduces 100% what the pages job produces: https://project.pages.drupalcode.org/gitlab_templates/help/documentation...

Sorry I did not see that open issue, I would have posted there. I have now added a comment on that issue.