docs: add API usage examples for NotifyChain workflows

[pelumixxril]

Overview

This PR adds practical API usage examples for common NotifyChain workflows, giving developers copy-paste-ready snippets for creating, scheduling, querying, and monitoring notifications, plus guidance on handling API errors.

Related Issue

Changes

📚 API Usage Examples

• [ADD] docs/API_USAGE_EXAMPLES.md
  • Creating notifications — request/response example for sending a notification.
  • Scheduling notifications — example of scheduling a notification for a future time.
  • Querying event history — example of retrieving past notification events.
  • Checking delivery status — example of looking up the delivery status of a notification.
  • Handling API errors — example error responses and recommended handling patterns.

Verification Results

Docs-only change:
✅ Examples reviewed for consistency with existing API conventions
✅ All five requested workflows covered (create, schedule, query, status, errors)

Acceptance Criteria	Status
Creating notifications example included	✅ Documented with request/response
Scheduling notifications example included	✅ Documented with request/response
Querying event history example included	✅ Documented with request/response
Checking delivery status example included	✅ Documented with request/response
Handling API errors example included	✅ Documented with error responses and handling guidance

Closes #868

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