docs/Brew-Bundle-and-Brewfile: document Flatpak support

[botantony]

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[Copilot]

Pull request overview

This PR documents the newly added Flatpak package support in Homebrew Bundle by updating the Brew-Bundle-and-Brewfile documentation. The changes integrate Flatpak into the existing documentation framework, showing users how to declare Flatpak packages in their Brewfiles.

Key Changes

• Added three Flatpak examples demonstrating different usage patterns (basic, with remote and URL, with remote only)
• Updated the "Adding New Packages Support" section to include Flatpak in the list of supported package managers

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[MikeMcQuaid]

Thanks!