docs(cli): add blog on unified training entry with Primus CLI

[Xiaoming-AMD]

This PR adds a short blog-style document describing the design philosophy
and practical advantages of using Primus CLI as a unified entry point for
training workflows on ROCm.

The document focuses on:

• why a unified CLI matters for large-scale training
• preserving execution semantics across local, container, and Slurm runs
• treating the CLI as part of the training system

This is a documentation-only change and does not affect runtime behavior.

No functional changes.

[Copilot]

Pull request overview

This PR introduces a technical blog post documenting the design philosophy and benefits of Primus CLI as a unified entry point for training workflows on ROCm. The document explains how a consistent CLI interface addresses common challenges in large-scale model training across different execution environments.

Changes:

• Added comprehensive documentation explaining Primus CLI's unified approach to training workflows
• Described how the CLI maintains consistent execution semantics across local, container, and Slurm environments
• Outlined practical benefits including improved reproducibility, simplified debugging, and reduced maintenance overhead