Improve language of user guide

[gagb]

This pull request improves the documentation for the AssistantAgent and teams features in the python/packages/autogen-core/docs/src/user-guide/agentchat-user-guide/tutorial/ directory. The changes enhance clarity, readability, and organization of the content. The most important changes include rephrasing sentences for better comprehension, restructuring sections, and adding new headings for better navigation.

Improvements to AssistantAgent documentation:

• Clarified the description of the AssistantAgent's capabilities.
• Added a new heading for the section on getting responses and improved the explanation.
• Changed the heading for the streaming messages section to be more consistent.
• Added context to the example code comments for better understanding.
• Improved the explanation of the on_messages_stream method and tool usage.
• Rephrased the transition to the next section for better flow.

Improvements to teams documentation:

• Reorganized the introduction to creating and running teams, and removed redundant information.
• Changed section headings for better consistency and readability. [1] [2]
• Improved the explanation and structure of the reflection pattern example.
• Added a new section on controlling a team and moved relevant content for better organization.
• Changed headings and rephrased content for better clarity in the sections on resuming tasks and pausing for user input. [1] [2] [3] [4]

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