docs(alerting): add unified alerts view page to starlight docs

[jackiehanyang]

Summary

• Add new Starlight doc page at /docs/alerting/unified-alerts-view/ describing Alerts Manager tabs and workflows.
• Add Unified Alerts View into Alerting navigation.
• Add cross-links from Alerting, Anomaly Detection, and Forecasting pages to the new unified page.

Testing

• Docs content and links verified by local diff review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.62%. Comparing base (20c5da9) to head (a223d8e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #431   +/-   ##
=======================================
  Coverage   55.62%   55.62%           
=======================================
  Files           4        4           
  Lines         169      169           
  Branches       47       47           
=======================================
  Hits           94       94           
  Misses         74       74           
  Partials        1        1           

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