fix(docs): include group in checkpoint loading paths

[CharlesXu-HQ]

Description

Problem

The checkpoint-loading examples in the English and Chinese FAQs still omit the optional group path segment introduced consistently by #562. When group is configured, these examples point users to the wrong checkpoint directory.

Changes

• Use ${checkpoint_root_dir}/${project}/${group}/${name} in all checkpoint conversion examples.
• Include group in the direct-loading Python examples.
• Explain that the group path segment is omitted when empty.
• Keep the English and Chinese FAQ guidance semantically aligned.

Testing

• Ran pre-commit checks on both modified FAQ files.
• Built the English and Chinese Sphinx documentation with ./build_doc.sh --branch fix/docs-checkpoint-group-paths.
• Confirmed the old command paths and three-component Python path are absent from both the source Markdown and generated HTML.
• Docstrings are not applicable because this PR does not change Python APIs.

Checklist

Please check the following items before code is ready to be reviewed.

• Code has passed all tests
• Docstrings have been added/updated in Google Style
• Documentation has been updated
• Code is ready for review