docs(readme): document extra-trusted-substituters and px0 usage

[Hetav21]

Summary

Update README.md to document extra-trusted-substituters in flake configuration snippets and include px0 in the declarative Usage example.

Changes

• Add extra-trusted-substituters = [ "https://hetav21.cachix.org" ]; to the flake nixConfig documentation and usage example.
• Add pkgs.nur.repos.hetav21.px0 to environment.systemPackages in the usage example.

Verification

• nix flake check passed.

[Hetav21]

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