docs: supplement key resources to Learn more section

[repeat-Q]

Description

Supplement 3 key resources (API Reference, Contributing Guide, Code of Conduct) to the "Learn more" section of the README, from LangChain's official repository.

Motivation

The existing section lacks API query, contribution guidance, and community norm resources. These supplements help users/contributors access comprehensive information without switching repositories.

Checklist

• Consistent with existing style
• All links are valid
• No duplicate content

[Copilot]

Pull request overview

This PR supplements the "Learn more" section of the README with three additional resource links: API Reference, Contributing Guide, and Code of Conduct. These additions provide users and contributors with direct access to key documentation that was previously missing from this centralized resource list.

Key Changes:

• Added API Reference link pointing to Python API documentation
• Added Contributing Guide link with information for new contributors
• Added Code of Conduct link to community guidelines

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[repeat-Q]

Hi maintainers! 👋
I was inspired by the "Additional resources" section in the main LangChain repository and noticed these key resources (API Reference, Contributing Guide, Code of Conduct) are very valuable for users and contributors. I thought supplementing them to the "Learn more" section here would make the README more comprehensive—letting people access all core resources in one place without switching repositories.
All supplemented content and links are directly from the main repo, ensuring consistency with official information. Hope this modification adds value to the project and can be merged smoothly! Thank you for your review!