Docs: Improve index.md with comprehensive feature overview

[ChenYi015]

Purpose of this PR

This PR significantly improves the main documentation page (docs/index.md) to provide a clearer overview of Arena's capabilities.

Proposed changes

• Rewrite the main description to better explain Arena's purpose as a CLI for ML workloads on Kubernetes.
• Add 'Key Features' section with organized subsections:
  • Simplified Training Management (TensorFlow, PyTorch, MPI, Spark, Ray, etc.).
  • Model Serving & Inference (TensorFlow Serving, Triton, KServe, KFServing, etc.).
  • Resource Management (GPU monitoring, node management, auto-scaling).
  • Model Lifecycle (registration, tagging, training/serving integration).
• Update Platform Support section with current framework list.
• Add 'Getting Help' section with links to documentation resources.
• Improve overall documentation structure and readability.

Change Category

• Documentation update

Rationale

The original index.md had a fragmented structure with unclear organization. This update provides a more modern, scannable documentation format that helps users quickly understand Arena's capabilities.

[google-oss-prow]

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