test: figure out how to enable the merge-queue in elastic/observability-robots-playground

[v1v]

What

Add docs PR builds for https://github.com/elastic/observability-robots-playground

Why

I'm figuring out how the buildkite-pr-bot works for the Organization so I can support the case of enabling the merge-queue for the APM Server and other projects.

For that reason, I only want to enable the docs-builds in https://github.com/elastic/observability-robots-playground without actually building anything, but I want to exit as soon as the repository is cloned.

Part of https://github.com/elastic/observability-robots/issues/2009

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