docs: document GCP service account workload identity connection

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Summary

Documents the new gcpServiceAccount service connection, which binds a Porter service to a GCP IAM service account via GKE Workload Identity so pods can call GCP APIs without managing key files.

Changes

• Replaced the "coming soon" GCP section in Secure cloud access with a full how-it-works guide, dashboard walkthrough, and porter.yaml accordion.
• Added a GCP Service Account Connection entry to Connections in porter.yaml with prerequisites, field reference, and example.
• Added a gcpServiceAccount example to the connections section of the porter.yaml reference.
• Added the new connection row to the connection types table and the GCP workload identity link to the related-docs list.

Context

Triggered by porter-dev/code#5774, which introduced the GKE Workload Identity connection type end-to-end (proto contract, API, dashboard config, reconciler) under the new gcpServiceAccount connection name with a single serviceAccountEmail field. Porter creates the roles/iam.workloadIdentityUser binding on the IAM service account at deploy time; users must pre-provision the service account and grant it the IAM roles it needs.

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Project	Status	Preview	Updated (UTC)
porter	🟢 Ready	View Preview	May 26, 2026, 10:52 PM