Add GCP self-hosted architecture documentation

[DavoCoder]

Overview

Adds documentation for self-hosting LangSmith on Google Cloud Platform (GCP), following the same structure and tone as the existing AWS and Azure self-hosted pages.

Changes

• New documentation page: docs/src/langsmith/gcp-self-hosted.mdx

  • Reference architecture with GCP services (GKE, Cloud SQL, Memorystore, Cloud Storage, Vertex AI)
  • Google Cloud Well-Architected Framework best practices (6 pillars)
  • Security, compliance, and monitoring guidance
  • References to GCP Terraform modules

• Navigation update: Added GCP page to docs/src/docs.json under "Self-hosted cloud architecture" section

• Architecture diagram: Added gcp-architecture-self-hosted.png (created separately)

Related

• Terraform modules: (PR pending review: Adding terraform + helm for GCP terraform#30)
• Follows same structure as AWS and Azure self-hosted documentation

Type of change

Type: New documentation page

Related issues/PRs

• Terraform modules: (PR pending review: Adding terraform + helm for GCP terraform#30)

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

Additional notes

This PR is dependent on (langchain-ai/terraform#30) to avoid a broken link

[katmayb]

LGTM from a docs perspective