docs: add scan-entities API documentation

[devin-ai-integration]

Add scan-entities API documentation

This PR adds documentation for the new scan-entities endpoint that allows paginated access to blueprint entities.

Changes

• Added new scan-entities.api.mdx documentation file
• Updated sidebar.ts to include the new endpoint under Entities category
• Documented pagination mechanism using cursor-based approach

Testing

• Documentation follows existing patterns and formatting
• Pre-commit hooks passed successfully

Link to Devin run: https://app.devin.ai/sessions/af7f1c4667ac4c4ebbd1a94072c7ea51

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