Create catalog-info file

[elastic-backstage-prod[bot]]

If you already have a ./catalog-info.yaml file defined in your repository, you probably want to close this PR.
If not - this PR provides you with the scaffolding for one. The catalog-info.yaml file is where you can define your infrastructure needs (such as Buildkite pipelines) in the form of Real Resource Entities.

If you already have a Buildkite pipeline defined in the elastic/ci repository, you can copy its definition in this catalog-info.yaml file (see instructions).
Once the pipeline definition is ingested into Backstage, Terrazzo will emit a warning about a duplicate definition with a warning similar to:
🙈 Ignoring remote manifest in favour of local one: buildkite.elastic.dev/v1=>Pipeline=> your-pipeline
At this point, it is safe for you to delete the Pipeline definition from the elastic/ci repository.

[cla-checker-service[bot]]

❌ Author of the following commits did not sign a Contributor Agreement: 259a6e98d61142ea5d298a8f87b3cde6b1c61518

Please, read and sign the above mentioned agreement if you want to contribute to this project

[esenmarti]

Hi @pquentin . Could we submit this PR?

[pquentin]

We can't due to the CLA check. I'll resubmit it