Add, adapt and run workspaces-images-check-links.sh in upstream CI

The anchor customizing-workspaces is in the application dashboard.
Therefore, we have this beautiful error during downstreaming:
tools/devspaces/workspaces-images-check-links.sh
ERROR: user-guide does not contain the anchor: customizing-workspaces
As we don't have this check upstream, nobody received a warning when we removed that anchor, and that's fine.
For the future, let's run this check in upstream.