Properly Handle Expected use Cases

[ChristianTackeGSI]

• We should document all our expected use cases.
• We should also make sure they work (maybe add CI jobs for them?).
• We should make sure, that they're reasonably simple to use.

Major use case categories:

• normal user install
• developer build of one package (spack dev-build)

Childs:

• Document and test classic "cmake/make/install" developer workflow #274
• Document and test spack "dev-build" developer workflow #275

[ChristianTackeGSI]

Part of the developer use case should have been addressed by #265.

[ChristianTackeGSI]

Moved to "next" milestone. Not going to handle spack related stuff NOW.