Add error handling documentation for Forge Realtime methods

What's changing We've added new error handling documentation for Forge Realtime. This covers expected error patterns and recommended handling strategies for all realtime operations:

• publish
• subscribe
• signRealtimeToken

The new guidance includes details on:

• Rate limit errors: How to identify and respond when your app exceeds Realtime service limits.
• Token pre-validation errors: Troubleshooting issues during the token signing and validation process.
• Common failures: A catalog of other frequent error scenarios with actionable recovery steps.

What you need to do Review the new https://developer.atlassian.com/platform/forge/realtime/error-handling-for-realtime-methods/ documentation to ensure your app gracefully handles these scenarios. We recommend implementing the suggested retry logic and validation checks to improve your app's reliability.