docs: add pkg-config to macOS prerequisites

[letv1nnn]

Summary

The Z3 installation section mentions pkg-config but doesn't list it as a required install on macOS. Adds it to the brew install line.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[github-actions]

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[letv1nnn]

I have read the DCO document and I hereby sign the DCO.

[letv1nnn]

recheck

[elezar]

As a matter of interest, is this already included for the other platforms?

[letv1nnn]

Yes, it's preinstalled on Ubuntu and Fedora. macOS is the only platform that needs it explicitly.