document contract events listening pattern

### Problem
the documentation doesn't provide clear guidelines for choosing between using mirror node or alternatives (like The Graph) for contract event listening.

### Impact
developers are forced to create fragmented workflows and write extra code to get event data reliably.
Required solution: Create a comparison of different methods for event listening, with sample code, pros/cons, and troubleshooting tips for each approach.

### Solution
create a comparison of different methods for event listening, with sample code, pros/cons, and troubleshooting tips for each approach.