Plugin guide enhancements

[schiwekM]

• Improved clarity of Node.js plugin section
• Added hint for modelling best practices to Java plugin section
• Added more samples to node.js plugin section
• Added warning for cds.connect.to within lifecycle events
• Added section for performance implications of handlers, and implications for data modification via after handlers
• Added section for req.data and req.query to hint at flexibility of these two parameters