Getting started with Maxtext with Cluster toolkit doc

[darisoy]

Description

This PR adds a comprehensive user guide (docs/run_maxtext/run_maxtext_via_cluster_toolkit.md) for orchestrating MaxText workloads on Google Kubernetes Engine (GKE) using Google Cloud Cluster Toolkit (gcluster), along with navigation links in docs/run_maxtext.md.

Historically, MaxText documentation only covered XPK (AI-Hypercomputer/xpk) and Pathways for large-scale GKE training orchestration. This PR introduces an end-to-end, verified guide for Cluster Toolkit (gcluster) as an evolving alternative for submitting multi-host JAX workloads via Kueue and JobSet.

Tests

Doc previewed.

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.