docs: enhance README with key features and practical examples

[crStiv]

Improve README.md documentation with the following enhancements:

• Add Key Features section for quick overview of AG2 capabilities
• Include navigation links for better document accessibility
• Restructure "What is AG2" section with Core Concepts
• Add practical code examples for basic usage scenarios
• Improve overall document structure and readability

These changes make the documentation more user-friendly and
help new contributors quickly understand AG2's capabilities.

[sonichi]

@crStiv thanks for the contribution. There's some wording I'd like to change. Could you invite me to the repo? Also, would you like to join our Discord server: discord.gg/ag2 and become a verified dev?

[crStiv]

@crStiv thanks for the contribution. There's some wording I'd like to change. Could you invite me to the repo? Also, would you like to join our Discord server: discord.gg/ag2 and become a verified dev?

@sonichi yes sure, I sent you an invitation.

[sonichi]

@crStiv thanks for the contribution. There's some wording I'd like to change. Could you invite me to the repo? Also, would you like to join our Discord server: discord.gg/ag2 and become a verified dev?

@sonichi yes sure, I sent you an invitation.

Thanks. Feel free to reach out to me on Discord: sonichi

[CLAassistant]

All committers have signed the CLA.