docs: improve quick start guide

[chenjiahan]

Summary

Improve quick start guide:

• fix incorrect sample code for Rsbuild.
• add example for RSDOCTOR=true npm run build.
• fix some small mistakes.

Checklist

• [ ] I have added changeset via pnpm run change.
• [x] I have updated the documentation.
• [ ] I have added tests to cover my changes.

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