Docs: write for users, not maintainers

[fstubner]

First pass of the copy review: the docs pages were partly written to the maintainer.

• Removed the pages' notes about their own past corrections (result model, core library, interface coverage), design rationale (the black-window section, packet capture, checksums, Packet Capture visibility), and build-pipeline wording ("where the release workflow publishes them", "depending on the release", "clap", "compile-time feature", "OUI").
• Moved the contributor rules out of the public Core library page. Ownership and compatibility rules were already in docs/ARCHITECTURE.md; the "adding a network capability" steps are added there. The page now links to it.
• Added the macOS and Linux desktop installs (Homebrew cask, .dmg, .deb, AppImage, AUR), and the macOS first-launch step (Open Anyway; right-click → Open stopped working in macOS 15).
• Fixed the 0.3.3 Windows signing claim (the app inside the MSI is signed from 0.3.4), the Packet Capture "greyed" vs "opens with guidance" contradiction (checked against App.tsx and PcapUnavailableState.tsx), and the desktop page contradicting itself on mDNS.
• Updated pages that still described TCP-only scans or an inspect without the OS hint, and "JSON/YAML" where CSV and Markdown exist too. One word, "interfaces", where interface coverage said "surfaces".

Not in this PR: re-capturing the JSON examples (CLI, MCP) from the released 0.3.4 build, which will include the protocol field. That follows the release. The landing-page copy changes are in #472.

The AI-prose Vale style passes on all eleven pages before and after.

[github-actions]

Site preview: https://pr-494.netscli-site-preview.pages.dev

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