docs: document Qwen3-Next balance serve build

[zcxGGmu]

Summary

• Add the required USE_BALANCE_SERVE=1 bash ./install.sh build step to the Qwen3-Next tutorial.
• Explain that ModuleNotFoundError: No module named 'sched_ext' means the balance_serve scheduler extension was not built in the active environment.

Refs #1514

Test Plan

• python documentation guard check for the new Qwen3-Next balance_serve build guidance
• git diff --check HEAD~1..HEAD

[greptile-apps]

Greptile Summary

This PR adds inline build steps for enabling the balance_serve extension (USE_BALANCE_SERVE=1 bash ./install.sh) to the Qwen3-Next tutorial, and adds a troubleshooting note for the ModuleNotFoundError: No module named 'sched_ext' error that appears when the extension was not compiled.

• The install block (git clone → cd ktransformers → git submodule update → USE_BALANCE_SERVE=1 bash ./install.sh → cd ..) is technically correct; the trailing cd .. restores the working directory so Section 4's python ktransformers/server/main.py invocation works from the parent directory.
• The original Installation Guide link is preserved as a prerequisite cross-reference, and the troubleshooting paragraph points users at the likely root cause of the missing module error.

Confidence Score: 5/5

Documentation-only change that adds a missing build step and a troubleshooting note; no code is affected.

The change is confined to a single markdown file. The added shell commands are technically correct, the working-directory issue from the prior review is resolved with cd .., and the Installation Guide link is preserved. No logic, configuration, or runtime behavior is touched.

Files Needing Attention: No files require special attention.

Important Files Changed

Filename	Overview
doc/en/Qwen3-Next.md	Adds USE_BALANCE_SERVE=1 build block and sched_ext troubleshooting note to Section 3; working-directory and Installation Guide link concerns from prior review are both addressed.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Reader starts Qwen3-Next setup] --> B[Section 2: Download model via huggingface-cli]
    B --> C[Section 3: Check prerequisites via Installation Guide]
    C --> D["git clone + git submodule update --init --recursive"]
    D --> E["USE_BALANCE_SERVE=1 bash ./install.sh"]
    E --> F{Build succeeded?}
    F -- Yes --> G["cd .. (restore parent dir)"]
    F -- No / ModuleNotFoundError: sched_ext --> H[Re-run install in clean environment]
    H --> E
    G --> I["Section 4: python ktransformers/server/main.py --backend_type balance_serve"]
    I --> J[Section 5: curl /v1/chat/completions]

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_{Reviews (2): Last reviewed commit: "[docs]: align Qwen3-Next install flow" | Re-trigger Greptile}

[zcxGGmu]

Thanks for the review — I pushed 65e9d45c79f8 addressing the Qwen3-Next install flow feedback.

Changes made:

• Restored the Installation Guide cross-reference before the inline build steps.
• Added cd .. after the source install so the following server launch command runs from the expected parent directory.

Validation:

• python content guard — passed, confirmed the prerequisite link and working-directory step are present.
• git diff --check — passed.