nixos docs: allow displaying package references

This is an improved version of original #12357.
For the purpose of generating docs, evaluate options with each derivation
in `pkgs` (recursively) replaced by a fake with path "\${pkgs.attribute.path}".
It isn't perfect, but it seems to cover a vast majority of use cases.
Caveat: even if the package is reached by a different means,
the path above will be shown and not e.g. `${config.services.foo.package}`.

As before, defaults created by `mkDefault` aren't displayed,
but documentation shouldn't (mostly) be a reason to use that anymore.

Note: t wouldn't be enough to just use `lib.mapAttrsRecursive`,
because derivations are also (special) attribute sets.