* 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
cbb5c53557