[DOCS] Standardize docstrings and add usage examples in apps/common/utils.py

### Description
While exploring the `backend/apps/common/utils.py` file, I noticed that several core utility functions (such as `round_down`, `natural_number`, and the case converters) provide brief descriptions but lack usage examples and detailed argument context.

To improve the Developer Experience (DX) for new contributors and align the codebase with the **Google Python Style Guide**, I propose updating the documentation for these utilities.

### Proposed Changes
I will refactor the docstrings in `backend/apps/common/utils.py` to include:
1. **Standardized Google-style formatting** for Args and Returns.
2. **Doctest-style Examples** (`>>>`) to provide immediate clarity on function behavior.
3. **Enhanced descriptions** for complex logic like `validate_url` and `natural_date`.

### Impact
This change will make the utility layer self-documenting and easier to maintain without adding any overhead to the runtime logic.

### Requirement
I would like to work on this issue. Please assign it to me!