docs: add reference documentation for docker.utils module

[Piyush0049]

Fixes #3386

Context

The docker.utils.parse_env_file utility function was added in version 1.4.0, but it was never documented in the Sphinx reference files. As a result, the changelog link to the docs has been pointing to an empty location, leaving users unaware of how to officially parse .env files using docker-py.

Furthermore, kwargs_from_env (the backbone of docker.from_env()) was completely missing a docstring.

Changes Made

• Added detailed Google-style Python docstrings with usage examples to both kwargs_from_env and parse_env_file in docker/utils/utils.py.
• Created a new docs/utils.rst page using the .. autofunction:: directive to automatically pull the documentation.
• Registered the new utils reference page in the docs/index.rst toctree.

Verification

• Built the Sphinx documentation locally (make docs) successfully.
• HTML renders cleanly without any warnings related to the newly added docstrings.