Use Compose Spec links in docs

[pedorich-n]

Since arion supports both docker and podman as a backend, I thought docs might point to a generic compose-spec because both of these backends implement it: https://github.com/compose-spec/compose-spec#implementations

I tested the docs with:

less $(nix build .#generated-option-doc-arion --print-out-paths)

I didn't test the validity of all the links. I clicked a couple and ensured the general way of constructing the link worked.

[pedorich-n]

@roberth are there any plans to get this merged? Or is there not much reason to have this?

[roberth]

Just didn't get around to it. Thank you!