Docs: Update JT docs to add a section about performance

[manzoorwanijk]

Proposed changes:

• Add a performance improvement section to JT docs
• Replace the deprecated FG link with the updated one

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[Copilot]

Pull Request Overview

This PR updates the Jurassic Tube (JT) documentation by adding a new performance improvement section and fixing a deprecated internal reference link.

• Adds a comprehensive performance optimization guide for tunneling with Caddy reverse proxy
• Updates the internal P2 reference link from PCYsg-snO-p2 to PCYsg-GJ2-p2

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[simison]

Suggested minor edits but just merge when you're happy with it :-)

Thank you for adding this!