Improve Contributing Guidelines Link and Add Community Support Section to Documentation Addresses #408

[divysinghvi]

Summary

• Fixed the broken Contributing Guidelines link in the documentation so it works on both GitHub and the documentation website.
• Added a "Community & Support" section to the contributing guidelines, including the Google AI Forum for Gemma, to help new contributors connect and get support.
• Improved the documentation navigation for better discoverability of contribution resources.

Motivation
The previous link to the contributing guidelines did not work on the documentation website because the file was not included in the docs build. This update ensures contributors can always access the guidelines and find community support.

Details
Updated the link in the contributing section to use an absolute URL.
Added a community section to the contributing guidelines.
Enhanced the documentation navigation for easier access to contribution information.Addresses issue #408

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