docs: Enhance core primitive documentation with Tracing and user-friendly definitions

[MuhammadHamidRaza]

This Pull Request significantly improves the "Core Concepts" section of the documentation, providing a more comprehensive and accessible overview of the SDK's primitives.

The existing documentation for the core primitives was good but could be more descriptive for new users. This update makes the following key changes:

• Added Tracing & Observability: This critical primitive is now included in the list of core concepts. Its description explains its purpose for debugging complex workflows and optimizing performance.
• Enhanced All Definitions: The descriptions for all five primitives—Agents, Handoffs, Guardrails, Sessions, and Tracing—have been rewritten to be more detailed, clear, and beginner-friendly. The new definitions explain not just what each primitive is, but also its purpose and how it directly benefits the developer.
• Improved Structure: The content is now organized with a more engaging introductory paragraph and well-defined bullet points, making it easy for developers to quickly grasp the fundamental concepts.

These changes make the documentation more thorough and helpful for developers of all skill levels. I believe this update will significantly improve the initial understanding and onboarding experience for all users of the OpenAI Agents SDK.

[seratch]

Thanks for sending this, but we would like to avoid making the top page wordier. Thanks again for your interest and time to make this SDK's documentation better!