BUG(docs): User Experience - With A2A Key Concepts Page

### What happened?

Multiple issues to be addressed in Key Concepts page.

**Page URL:** https://a2a-protocol.org/latest/topics/key-concepts/
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* **Problem:** The core concepts diagram is helpful but feels incomplete(feels outdated) and does not answer all questions a user might have after taking a course on the subject.
* **Proposed Fix:** Use a schema diagram to show connections between different elements, add a lifecycle flowchart, and clarify who initiates requests.
* **Justification:** This will create a more comprehensive and informative diagram that better explains the core concepts of A2A.
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* **Problem:** The term [Agent Response](https://a2a-protocol.org/latest/topics/key-concepts/#agent-response-task-or-message) is confusing in the context of A2A, which is a communication protocol between a server and a client.
* **Proposed Fix:** Use more specific terms like `A2A Client` or `A2A Server`.
* **Justification:** This will improve the clarity and precision of the documentation.
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* **Problem:** No mention of "Agent Skills" in Key Concepts page.
* **Proposed Fix:** Introduce the concept of Agent Skills earlier in the documentation.
* **Justification:** This will provide a more logical learning path for new users.

### Relevant log output

```shell

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

L’issue renvoie vers la page Key Concepts à l’adresse https://a2a-protocol.org/latest/topics/key-concepts/. Commencez par localiser les fichiers sources de cette documentation, probablement dans un répertoire docs/ ou website. Examinez le diagramme et le texte existants. Les tâches consistent à mettre à jour les diagrammes (un outil de diagramme peut être nécessaire), à clarifier la terminologie, par exemple en remplaçant 'Agent Response' par 'A2A Client' ou 'A2A Server', et à ajouter une section sur 'Agent Skills'. Vérifiez s’il existe des directives de contribution à la documentation. 'Done' signifie que la page a été mise à jour avec les corrections et les diagrammes proposés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.