Fix documentation and tests for horizon.packageSet module

This MR adds validation and improves documentation for the horizon.packageSet module.

Changes

• Required packages validation: Adds checks.required-packages that validates packages listed in requiredPackages are present in the package set

• Simplified README template: Removes horizon-shell documentation section from the README generator

• Test improvements: Updates test flakes to demonstrate package definition with cabal2nix-generated semigroups package

• Golden test READMEs: Adds README.md files to test flakes that serve as golden tests for the automatic README generation

Example of the validation check

When requiredPackages = [ "semigroups" ] is configured, the check ensures the package exists:

$ nix build .#checks.x86_64-linux.required-packages
# Passes if semigroups is in packages, fails with clear error otherwise

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