Improve staging site to aggregate recent PRs

[LukasWallrich]

@richarddushime as discussed - could you rework the staging site so that it includes all (non-draft!) PRs that are open unless no merge conflicts exist? For that, it would probably make sense to first pull the latest from main into each PR, so that there are no spurious conflicts, then merge all PRs into a dev branch (starting from the latest, until a merge conflict stops this) and then deploy that. It would then be important to output a clear message saying which PRs are included.

In addition, we should keep the current approach as a manual option to deploy a single PR - in case the most recent breaks the build while we want to check out another one.

No rush on this - and if it it turns out to be too fiddly, you can also close it, but it would make review easier.

[github-actions]

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