Update create-channel.md with popup/mobile instructions

[okwme]

I struggled with this so maybe someone else will too.

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PR-Codex overview

This PR enhances the documentation for the create-channel.md file by providing specific instructions for handling link presentations for desktop and mobile users, including code snippets for opening URLs in popups.

Detailed summary

• Added instructions for desktop users to present links as popups with a QR code.
• Included code snippet for opening a URL in a popup with specified dimensions.
• Provided instructions for mobile users to open links directly in the app using _top.
• Included a corresponding code snippet for mobile URL handling.

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[vercel]

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