docs: add "Use within an agent" section to Notion integration

[Copilot]

This PR addresses the feedback from @mdrxy to add a missing "Use within an agent" section to the Notion integration documentation, following the standard pattern used across other LangChain tool integrations.

Changes Made

Restructured the agent documentation section:

• Renamed the "Chaining" section to "Use within an agent" to match the standard pattern
• Added proper introduction text referencing the agent tutorials: "Below we show how to incorporate the toolkit into an agent"
• Added ChatModelTabs component for LLM selection following the standard format
• Reorganized the agent example code into logical cells matching the Gmail integration pattern

Fixed documentation inconsistencies:

• Corrected "Discord API" typo to "Notion API" in the credentials section
• Updated installation command from langchain-community to langchain-notion

Maintained backward compatibility:

• Preserved all existing functionality and examples
• Kept all langchain_notion imports and usage patterns
• Maintained the same agent demonstration with NotionWrapper and LangchainNotionToolkit

Validation

The documentation now follows the exact same structure as the Gmail integration referenced by the maintainer, including:

• Standard section headers and organization
• ChatModelTabs integration for LLM selection
• Proper agent tutorial references
• Clean separation of setup, instantiation, and usage examples

The JSON structure has been validated and all essential functionality is preserved while bringing the documentation in line with LangChain standards.

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