Add NRP use cases/scenarios

[dmiksik]

According to https://docs.nrp.eosc.cz/en/docs/repo_admins/operating-repositories-in-the-nrp/conditions-for-creating-repositories#4-basic-use-cases-of-nrp, there are 3 main scenarios for establishing a repository in the NRP:

1. NRP resources + core repository system
2. NRP resources + other repository system
3. integration of an existing independently operated repository

IMO, the respective scenario should be stated explicitly for each repository. (And there will arise hybrid scenarios, as always, co adding new scenarios should be possible without breaking things.)

[peterlenyi]

I upvote this. Having this information is useful for the repository-AAI integration, too, as we follow a slightly different template.