Rewrite notification-listener article

- Rewrite intro, remove smartwatch/phone references
- Update namespace note to dual-namespace pattern
- Remove Windows 10 version check section
- Remove Build 17763 known issue warning
- Remove 'How to fix delays' section (Windows Mobile era)
- Rewrite background task section with WASDK BackgroundTaskBuilder
  and UserNotificationChangedTrigger, link to background tasks article
- Clean up wearable references throughout
- Fix non-breaking spaces
- Add See also with overview link

Co-authored-by: Copilot <[email protected]>

Author: drewbatgit