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

Introduces comprehensive contributor docs for deep links and surfaces them in the README.

• Adds docs/deep-links.md covering overview, example behavior, user flow (mermaid), creating/signing links via branch.io, extension route registration (pathname, getTitle, handler), architecture class diagram, deferred deep links, and security notes
• Updates README.md Table of Contents to include Deep links Guidelines pointing to docs/deep-links.md

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[davidmurdoch]

Just a quick pass. I'll need to read more in depth later.