docs: add Bonito Niri quick-start

[hanthor]

What changed

• Add docs/niri-quickstart.md for using the Bonito Niri flavor.
• Explain Niri's scrollable-tiling model and provide a keybindings table based on the upstream default configuration.
• Document ~/.config/niri/config.kdl, validation/reload, launcher, gaps, and wallpaper customizations.
• Cover Flatpak and Homebrew apps, bootc updates and rollback, display scaling, screen recording, and troubleshooting.
• Add the page to the Getting Started docs navigation.

Why

Give new Niri users a practical day-one guide for the Bonito variant, including the controls and configuration concepts that differ from GNOME and KDE.

Validation

• git diff --check
• npm run build (passes; existing repository warnings remain for duplicate XFCE routes, deprecated config, and unrelated broken links)

Fixes #217