DeepSeek V3.2 user guide update

[snehalv2002]

Description

Updating the user guide for DeepSeek-V3.2. Explains new feature updates and updates instructions on multi-stage lightning indexer training and checkpoint conversion.

Tests

Verified maxtext/tests/end_to_end/tpu/deepseek/v3.2-671b/2_test_deepseek.sh before adding to XLML dag.
Command: https://paste.googleplex.com/5274574397767680
Logs: https://cloudlogging.app.goo.gl/d9HUWQLzQa74HzfB7

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 @RissyRan, 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 updates the DeepSeek user guide to include instructions for the new DeepSeek-V3.2 model, specifically focusing on indexer training and checkpoint conversion. The updates are timely and provide clear steps for users to leverage the latest sparse attention features.

🔍 General Feedback

• Consistency: Ensure that the model names (deepseek3.2-671b) and tokenizer paths (deepseek-ai/DeepSeek-V3.2) are consistent across all stages of the guide.
• Syntax: Be careful with trailing backslashes in shell command examples, as they can cause errors if users copy-paste the last line.
• Clarity: Using concrete example values (like 0.1 for scaling factors) is generally more user-friendly than placeholders in curly braces.

[RissyRan]

Thanks for your 1st PR!!!

One more thing, could you update the PR desperation to follow our default template? One example: here

[shuningjin]

Thanks for the update! Might be good to organize deepseek3.2 into a self-contained section for clarity, and add more explanation on continued pre-training for indexer.

[Rohan-Bierneni]

LGTM. Thanks for fixing this issue!

Approved the wrong pr by accident. Sorry for the confusion!

[RissyRan]

Please address minor comments.

I think decoding case is clear, shall we add announcement too here?

cc @shuningjin @Rohan-Bierneni

[shuningjin]

Thanks for the update! Left some minor comments.

[shuningjin]

Great work!!