Add role documentation, add system begin & end tokens

[objnf-dev]

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Motivation

Lack of documentation on how to use the role of Chat model, and lack of begin/end tokens for the System role.

Modification

Add role documentation, and system_begin(), system_end() function in api.py.

Checklist

1. Ensure pre-commit pre-commit run --all-files or other linting tools are used to fix potential lint issues.
2. Confirm that modifications are covered by complete unit tests. If not, please add more unit tests for correctness.
3. Modify documentation as needed, such as docstrings or example tutorials.

[Ying1123]

please fix the lint error

[objnf-dev]

Should be fixed now.

[Ying1123]

@objnf-dev Thanks for the contribution!