Add Spinnaker container plugin howto

[rake-ai]

Summary

Adds a how-to guide for the Spinnaker container plugin for Digital.ai Release.

Changes

• New container/spinnaker/README.md with complete setup and usage instructions
• Screenshots for connection configuration and all 5 task panels

Tasks covered

• Trigger Pipeline (Container) — trigger a Spinnaker pipeline and optionally wait for completion
• Get Applications (Container) — list all Spinnaker applications
• Get Pipelines (Container) — list pipelines for a given application
• Get Pipeline Config (Container) — retrieve the full configuration of a pipeline
• Get Pipeline Status (Container) — check the execution status of a pipeline run