Python: Clarifications on installing packages in README

[moonbox3]

Motivation and Context

Now that our package structure is set, updating the Python README to clarify how to install various packages/subpackages.

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull Request Overview

This PR updates the Python README to provide clearer guidance on package installation options. The changes help users understand when to use full framework installation versus selective package installation based on their specific needs.

• Restructures installation instructions into two distinct paths: development mode and selective install
• Adds explanatory context for when to use each installation approach
• Clarifies which components are included with each installation option

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