Document use case: Feature Resolution

[DavHau]

This PR adds documentation for the "Be able to depend on features of other packages (feature resolution)" use case. The document includes:

• General information about feature resolution in Nixpkgs and its importance
• A concrete example illustrating a scenarios where feature resolution is required
• An outline of current problems faced in Nixpkgs regarding feature resolution

With this document, we aim to highlight the challenges in feature resolution and provide a basis for further discussions and improvements in Nixpkgs.

[DavHau]

I'm merging this now and am happy to push further adjustments once feedback comes up.

[infinisil]

Also to mention here: https://github.com/nixpkgs-architecture/pkgs-modules/pull/1#issuecomment-1548277593