doc: add refererence (autodoc from docstring)

[jychp]

Summary

This PR enhances the maintainability of the codebase by clarifying function behaviors, harmonizing docstring formatting, and enabling developer tooling compatibility. It does not introduce any functional change.

Related issues or links

n/a

Changes proposed

• Added documentation to common utility functions and core modules
• Improved docstring consistency across the codebase (grammar, parameter descriptions)
• Improved style parsing in the documentation config to support both Google and NumPy docstring formats (alongside already supported RST format)

How to test this PR

No functional testing required — this PR only touches comments and documentation metadata.

Screenshots

Reviewer notes

• Focus is on docstring clarity and consistency; no functional logic has changed
• Style is aligned with the Google Python Style Guide to ease MyST/Markdown integration