Add Vertex AI workload identity setup guide (#5637)

* docs: add Vertex AI workload identity setup guide

Walks SaaS customers through configuring CrewAI AMP to authenticate to
Google Vertex AI via GCP Workload Identity Federation, eliminating the
need for long-lived service account keys.

* docs: restrict Vertex WI guide to v1.14.3+ navigation

The guide requires `crewai>=1.14.3`, so registering it under older
version snapshots is misleading. Keep the entry only in the v1.14.3
English nav.

* docs: clarify crewai-vertex SA name is an example