clickpipes: documentation for GCP-native private networking (PSC)

[morsapaes]

Summary

Work-in-progress for PSC integration documentation.

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Note

Low Risk
Documentation-only addition with no application or infrastructure code changes.

Overview
Adds a new integration guide at docs/integrations/data-ingestion/clickpipes/gcp-psc/postgres.md (slug /integrations/clickpipes/gcp-psc/postgres) documenting GCP Private Service Connect for Postgres CDC ClickPipes via Reverse Private Endpoints (GCP_PSC_SERVICE_ATTACHMENT).

The page explains how RPEs consume customer-published PSC service attachments, why custom_private_dns_mappings is required on GCP, and prerequisites including allowing the clickpipes-production consumer project. Path A walks through Cloud SQL native PSC (gcloud enable/allow list, reading pscServiceAttachmentLink and dnsName, RPE creation in UI/Terraform/API, then wiring a cloudsqlpostgres pipe). Path B covers producer-owned PSC (private-network Cloud SQL or self-managed Postgres) with the gcp-cloud-sql-private-network-psc Terraform module and optional manual PendingAcceptance approval.

It also documents RPE lifecycle in the UI, same-region constraints, and limitations (no VPC peering). Supported GCP regions still has a publish-blocking TODO to paste the region list; the UI steps repeat numbering at item 2.

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