Link Github to docs repo for easier contribution

[nounder]

PR-Codex overview

This PR updates the link for the github icon in the docs/.vitepress/config.mts file from the protocol repository to the docs repository.

Detailed summary

• Changed the link for the github icon from https://github.com/farcasterxyz/protocol to https://github.com/farcasterxyz/docs.

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[vercel]

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