doc: Add LoRa PHY packet format

[pradeeka7]

Summary

This page explains the PHY structure of the LoRa packet.

Screenshots

Changes

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Notes for Reviewers

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Checklist

• [X] Scope: The referenced issue is addressed, there are no unrelated changes.
• [X] Run Locally: Verified that the docs build using make server, posted screenshots, verified external links.
• [X] Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• [X] Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.