Update docs: clarification on pip authentication with setup-python

[v-priya-kinthali]

Description:
This pull request updates the documentation to clarify that setup-python does not manage pip authentication for private package sources.
Related issue:
#104

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull Request Overview

This PR updates the README to clarify that the setup-python action does not manage pip authentication for private package sources.

• Adds a note in the installation steps about pip authentication being out of scope for setup-python.
• Provides links for users to find pip-related documentation.

Comments suppressed due to low confidence (1)

README.md:97

• The linked 'VCS support' doc focuses on version control integration rather than authentication. Consider linking to the pip authentication/configuration docs (e.g., https://pip.pypa.io/en/stable/user_guide/#environment-variables) for guidance on credentials and private indices.

>The `setup-python` action does not handle authentication for pip when installing packages from private repositories. For help, refer [pip’s VCS support documentation](https://pip.pypa.io/en/stable/topics/vcs-support/) or visit the [pip repository](https://github.com/pypa/pip).