Add downstream configuration template

[lbarcziova]

Related to packit/packit-service#2301

This is just an idea/draft. Let me know WDYT

TODO:

• [x] think about better placement
• [x] reference from guide and maybe somewhere else
• [ ] revise the hints (and action commands)

[softwarefactory-project-zuul[bot]]

Build succeeded. https://softwarefactory-project.io/zuul/t/packit-service/buildset/2f35a4c8a03545c59a7f41b8e961b3b3

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Build succeeded. https://softwarefactory-project.io/zuul/t/packit-service/buildset/0bf5f8bff3ae4f20b788286c4233245a

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Build succeeded. https://softwarefactory-project.io/zuul/t/packit-service/buildset/a31882369a7c4d86bbaf6450b7e4adeb

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Build succeeded. https://softwarefactory-project.io/zuul/t/packit-service/buildset/67b05c6646fe408a9c91f81c816d11c9

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Build succeeded (gate pipeline). https://softwarefactory-project.io/zuul/t/packit-service/buildset/28b14c11b5144ed88272c795bfa59e4e

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