Add distillation reference guide and online-distillation recipe

[gagika]

Description

Adds a new reference guide (docs/guides/distillation.md) covering the online distillation trainer — loss anatomy, α/β/temperature schedules, layer-index selection, monitoring, and troubleshooting — and extends the knowledge-distillation tutorial with an Online Distillation section (Pattern A compression, Pattern B depth-pruning recovery, XPK multi-host, and the offline top-k logits variant). The new guide is linked from docs/guides.md.

Tests

Run most of commands.

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.

[github-actions]

🤖 Hi @gagika, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[github-actions]

## 📋 Review Summary

This Pull Request introduces comprehensive documentation for online distillation in MaxText, including a new reference guide and an expanded tutorial. It also adds Gemma 4 model details and configuration updates for MoE load balancing.

🔍 General Feedback

• The new distillation guide is very well-structured, covering architecture, loss anatomy, and practical tuning advice.
• Most documentation updates are accurate and include helpful examples for both single-host and multi-host (XPK) setups.
• There is a recurring pattern of backslash escaping for MyST directives in docs/guides.md that appears to be a regression and should be corrected to ensure proper rendering.

[github-actions]

🟠 The backslash escaping \{grid} and \{grid-item-card} is likely a mistake. In MyST markdown, directives are typically formatted as :::{directive}. Adding a backslash before the curly brace will likely cause the documentation builder to render the brace and directive name as literal text instead of parsing it as a directive. This appears to have been applied to all cards in this file.

Suggested change

	::::\{grid} 1 2 2 2
	:gutter: 2
	:::{grid-item-card} ⚡ Optimization
	:::\{grid-item-card} ⚡ Optimization
	::::{grid} 1 2 2 2
	:gutter: 2
	:::{grid-item-card} ⚡ Optimization

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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