Update geting started doc

[SurbhiJainUSC]

Description

This PR transitions the "First Run" tutorial (first_run.md) into a top-level "Getting Started" guide (getting_started.md).

Instead of hosting inline installation and run commands (which quickly become outdated or duplicated), the document now serves as a landing page. It guides users through prerequisites and then points them to the dedicated, detailed guides for installation, pre-training, post-training, inference, and notebook usage.

Tests

Locally build:
https://screenshot.googleplex.com/4RMpSB5gm3tJNcB
https://screenshot.googleplex.com/7zxFKPHfCDcFJxs

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.