docs: add man page generation via cobra/doc

[mendsec]

Context

Power users and package maintainers (Homebrew, AUR, .deb) expect Unix man pages. Cobra ships cobra/doc which generates man pages from command definitions. Add a cmd/catnet-docs/main.go generator and a Makefile target make man that outputs to docs/man/.

Acceptance criteria

• make man generates catnet.1, catnet-scan.1, catnet-export.1, catnet-version.1 in docs/man/
• Man pages include NAME, SYNOPSIS, DESCRIPTION, OPTIONS, EXIT STATUS, EXAMPLES sections
• GoReleaser archives include docs/man/ for Linux targets
• README Installation section mentions man page availability