Update blog 5.4: add Key Vault, managed identity, and secure settings

[workcontrolgit]

Summary

• Add Key Vault module walkthrough (Step 6) — RBAC authorization, soft-delete, role assignments
• Add managed identity section in Web App module (Step 3) — SystemAssigned identity, principalId output
• Add Step 7: Storing Secrets in Key Vault — az keyvault secret set commands
• Add Step 8: Using Key Vault References in App Service — @Microsoft.KeyVault(SecretUri=...) syntax
• Add Step 10: Deploy via GitHub Actions — workflow_dispatch workflow walkthrough
• Update file structure diagram to include keyVault.bicep
• Update resource count: 8 → 9 resources
• Update cost estimate: add Key Vault (~$0/month at dev scale)
• Update "What You'll Learn" with managed identity, Key Vault, and GitHub Actions workflow
• Update tags to include #keyvault #managedidentity #secretsmanagement #zerotrust