Clarify that packages are not handled by the autowiring

[madidier]

I was confused when I read the documentation section on autowiring, the wording seemed to imply that packages would be autowired, but by looking at the code, it seems not to be the case. I'm suggesting the following changes to make that clearer.

[srid]

Out of curiosity, what would it take to actually implement this?

[anntnzrb]

This documentation improvement identified a real gap in functionality. Instead of just documenting that packages aren't autowired, I've implemented the actual package autowiring feature! 🎉

✅ Implemented in PR #142: https://github.com/srid/nixos-unified/pull/142

The implementation:

• Automatically scans packages/ directory and wires packages as packages.${system}.{name} flake outputs
• Uses pkgs.callPackage for each package file
• Includes comprehensive documentation with examples
• Tested and working

This addresses @srid's question "what would it take to actually implement this?" by providing the actual functionality rather than just documenting its absence.

PR #142 should close this issue once merged, as it implements the missing feature that this documentation PR was trying to clarify.