docs: add token exchange prerequisites and improve examples

[xiaoyijun]

Summary

• Add prerequisites section for token exchange, explaining that users need to enable "Allow token exchange" in application settings
• Extract token exchange prerequisites into a reusable fragment
• Add machine-to-machine (M2M) application examples alongside traditional web applications
• Improve code examples with syntax highlighting for key lines
• Unify URL encoding style (use unencoded format for better readability)

Testing

• Verified markdown syntax is correct
• Checked that fragment imports work properly

Checklist

• .changeset
• unit tests
• integration tests
• necessary TSDoc comments

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