SPIKE: Research templating

[lachmanfrantisek]

We've got a couple of requests for config templating. Let's spend a while researching various options and use cases.

related issues:

• allow more templating in .packit.yml packit#2038
• MonoRepo: Templates packit#1925

The task of this is to create a new document and:

• Summarise the use cases -- e.g. monorepo (e.g. Copr or some layered distributions) or a couple of similar packages maintained by the same person (e.g. Forman).
• Suggest implementation.
• Cleanup/(re)define the existing issues. Create an epic if needed.