doc: expand and reorganize FAQ index page

[ChenYi015]

Purpose of this PR

Expand the FAQ index page to provide more comprehensive answers to common questions and improve the documentation structure for better user experience.

Proposed changes:

• Reorganized the FAQ into logical categories (Installation, Training, Serving, Common Issues, etc.).
• Added detailed Q&A sections for resource constraints, authentication, data/volume issues, and GPU issues.
• Included a Troubleshooting Workflow checklist.
• Added Performance Tuning guidelines for both training and inference.
• Updated related resource links.

Change Category

• Documentation update

Rationale

As Arena grows, users encounter a wider range of issues. A centralized, well-structured FAQ helps reduce support burden and empowers users to resolve common problems independently.

[google-oss-prow]

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