GitHub CI has a feature called "Environments"
which actually means "Deployment environments", such as the staging server.

Our test-and-deploy.yaml and test-and-deploy-ipv4only.yaml workflows are deployments,
so this feature fits perfectly. Same for docs.yaml and docs-preview.yaml deployments,
deployment of documentation is a deployment too.

The good thing about it is that it
natively supports concurrency limits
so we can get rid of the "jsok/serialize-workflow-action".

The other nice feature is that it is possible to specify the URL
for deployment, so you can jump directly to it from the PR (for documentation previews)
and from the front page (for "production" deployment of docs).

It has also other features like being able to move secrets into environment secrets
and allow only some users to trigger deployments,
but this is only interesting for production deployments
and not staging servers that everyone with write access should be able to use anyway.