docs: add beginner explanation for single agent example

[yukthagangadhari5]

Description of the change

Problem:
New users may find it unclear what an “agent” represents in ADK before encountering the first code example in the README.

Solution:
Added a short beginner-friendly explanation before the single-agent example to clarify what an ADK agent is and what the example demonstrates. This improves readability and onboarding without changing any existing behavior or code.

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Testing Plan

This change is documentation-only and does not affect runtime behavior. No tests were required or run.

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Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own change.
• I have commented my code, particularly in hard-to-understand areas. (Not applicable – documentation only)
• I have added tests that prove my fix is effective or that my feature works. (Not applicable – documentation only)
• New and existing unit tests pass locally with my changes. (Not applicable – documentation only)
• I have manually tested my changes end-to-end. (Not applicable – documentation only)
• Any dependent changes have been merged and published in downstream modules. (Not applicable)

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Additional context

This change is intended to improve the onboarding experience for users exploring ADK for the first time.

[google-cla]

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[gemini-code-assist]

Summary of Changes

Hello @yukthagangadhari5, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the ADK documentation by introducing a concise, beginner-oriented explanation of what an 'agent' represents within the framework. This addition, placed strategically before the initial code example in the README, aims to clarify fundamental concepts for new users, thereby streamlining their learning curve and improving overall readability without impacting any functional aspects of the system.

Highlights

• Documentation Enhancement: Added a beginner-friendly explanation to the README.md file, clarifying the concept of an 'agent' in ADK before the first code example.
• Improved Onboarding: The new explanation aims to improve the onboarding experience for new users by making the purpose and definition of an ADK agent more explicit.
• No Code Changes: This pull request is documentation-only and does not alter any existing code or runtime behavior of the application.

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[gemini-code-assist]

Code Review

This pull request adds a helpful explanation for beginners about what an 'agent' is in the ADK, right before the first code example. The change is well-intentioned and improves the onboarding experience. I've suggested a small formatting tweak to the new documentation to improve its readability and make the Markdown more robust for future edits.

[gemini-code-assist]

This is a great addition for new users! For better readability and more robust Markdown formatting, I suggest separating the note's title from the content with a blank line and combining the explanation into a single paragraph. This avoids using trailing spaces for line breaks, which can be fragile and harder to maintain.

Suggested change

	**Beginner note:**
	In ADK, an *agent* is a Python object that defines how your AI system behaves —
	including its instructions, the tools it can use, and how it responds to users.
	The example below shows the simplest way to define a single agent.
	**Beginner note:**
	In ADK, an *agent* is a Python object that defines how your AI system behaves — including its instructions, the tools it can use, and how it responds to users. The example below shows the simplest way to define a single agent.

[ryanaiagent]

Hi @yukthagangadhari5 , Thank you for your contribution! It appears you haven't yet signed the Contributor License Agreement (CLA). Please visit https://cla.developers.google.com/ to complete the signing process. Once the CLA is signed, we'll be able to proceed with the review of your PR. Thank you!

[yukthagangadhari5]

Hi Rohit, thank you for the message! I’ve already signed the Google Individual Contributor License Agreement earlier today. It may still be syncing with GitHub, but please let me know if there’s anything else needed from my side.

[yukthagangadhari5]

Thank you! I’ve now signed the Google CLA and the check is passing.
Please let me know if any further changes are needed.