Add Docs for CI Concepts and Use Cases

[chalindukodikara]

Purpose

New CI Documentation (docs/user-guide/ci/)

• Overview: CI architecture, execution lifecycle, and default workflows
• Component Workflow Schema: Parameter system and schema syntax
• Custom Workflows: Creating and customizing ComponentWorkflows
• Additional Resources: Defining ExternalSecrets and other resources for workflows

Private Repository Guides

• Private Git Repository: Configure authentication for private repos
• Multiple Private Git Organizations: Support multiple organizations with dynamic credential selection

External Registry Guide

• External Container Registry: Push built images to ECR, GCR, ACR, or other registries

Related Issues

openchoreo/openchoreo#1444

Checklist

• Updated sidebars.ts if adding a new documentation page
• Run npm run start to preview the changes locally
• Run npm run build to ensure the build passes without errors
• Verified all links are working (no broken links)