Fix local preview guidance for course chapters

[junepil]

Background

When contributors preview translated course content locally, doc-builder preview may start successfully but still return runtime 404 for valid routes.

Root Cause

The issue came from a dependency/version skew in the local tooling:

• Python package: hf-doc-builder installed from PyPI (v0.5.0) which generates .mdx file for preview.
• Frontend template: cached doc-builder kit from a newer revision which requires +page.svelte file per path.

Because of this mismatch, route generation behavior was inconsistent and local preview could boot while serving unresolved routes as 404.

What Changed

1. Updated contributor instructions in README.md to:
   • use a clean virtual environment;
   • install hf-doc-builder from the current GitHub source;
   • run preview with explicit --language LANG-ID.
2. Added a troubleshooting section for preview-time 404 that clearly points to re-installing hf-doc-builder and restarting preview.

Why This Helps

Contributors now get deterministic setup steps that keep the Python CLI and SvelteKit template in sync, which prevents route-generation mismatch and avoids false-negative 404 previews.

Verification Notes

• Confirmed translated routes render after aligning doc-builder dependency source.
• Confirmed chapter routes behavior is stable when preview is started with explicit language flag.

Things To Note

• Currently, the root path isn't configured properly due to the absence of root path document file.

References

• huggingface/course#1098
• huggingface/doc-builder#448

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[junepil]

@stevhliu If you have time please take a look, have a wonderful day 😎

[stevhliu]

nice, thanks for this! 🤗