Add documentation for deep links

[david0xd]

This PR adds contributor documentation for deep links.

Click here for rendered markdown version.

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Note

Adds deep links contributor documentation and links it from the README.

• Documentation:
  • Adds docs/deep-links.md detailing:
    • Overview, example behavior, and user flow (mermaid diagram) for deep links
    • Steps to create/sign links and register routes in the extension (with code snippets)
    • Architecture class diagram and security notes
• README:
  • Adds link to docs/deep-links.md in the table of contents

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