feat: add ADO to GitHub structural mapping documentation for migration guidance

Author: CalinL

README.md

@@ -55,6 +55,7 @@ Comprehensive L400-level technical documentation for GitHub Enterprise Cloud adm
 - [🔀 GitHub Enterprise Importer ADO Guide](docs/14-github-enterprise-importer-ado-guide.md) - Step-by-step guide for using GitHub Enterprise Importer with Azure DevOps
 - [� Azure Pipelines with GitHub Repos Integration](docs/15-azure-pipelines-github-repos-integration.md) - Impact analysis of using Azure Pipelines with GitHub repositories after migration
 - [🔍 Azure DevOps to GitHub Migration Analysis](docs/16-azure-devops-to-github-migration-analysis.md) - Detailed technical analysis of the migration process
+- [🗺️ ADO to GitHub Structural Mapping (Visual)](docs/ado-to-github-structural-mapping.md) - High-level Mermaid visuals mapping ADO concepts (Organization, Project, Repos, Teams, Security, Pipelines, Governance) onto GitHub's Enterprise/Org/Repo tiers; explains the project-based vs repo-based shift and the Red-Green-Sandbox target org model
 - [🔑 ADO REST API Authentication Without PATs](docs/ado-rest-api-auth-without-pat.md) - Alternatives to Personal Access Tokens for Azure DevOps REST API authentication in CI/CD pipelines
 - [🧹 ADO Tenant & Org Cleanup](docs/ADO-Tenant-Org-Cleanup.md) - Post-migration cleanup of Azure DevOps tenant connections, PAT policies, orphaned orgs
 - [❓ ADO to GitHub Migration Q&A Guide](docs/ADO-to-GitHub-Migration-QA.md) - Detailed questions and answers about migrating from Azure DevOps to GitHub using GEI