Add Azure security documentation and best practices for .NET Aspire deployments

[Copilot]

This PR adds comprehensive Azure security documentation for .NET Aspire deployments as requested in the issue. The new article covers:

What's included

New documentation: docs/deployment/azure/azure-security-best-practices.md

Default security posture

• Container-level security (secure base images, non-root execution, minimal surface)
• Network security (private networking, encrypted service-to-service communication, HTTPS endpoints)
• Identity and access management (managed identities, Azure RBAC integration)
• Configuration and secrets management (secure configuration, environment variables)

Additional security hardening

• Azure Key Vault integration with code examples
• User-assigned managed identities configuration
• Network security implementation (virtual networks, NSGs, private endpoints)
• Comprehensive monitoring setup (Application Insights, Azure Monitor)
• Data storage security (encryption at rest, backup retention)

Production considerations

• Environment separation strategies
• Secret rotation procedures
• Access control best practices (principle of least privilege, JIT access, MFA)
• Compliance and auditing (audit logging, vulnerability scanning)

Security monitoring and incident response

• Continuous monitoring setup
• Incident response planning and procedures

Changes made

1. Created new article at docs/deployment/azure/azure-security-best-practices.md (203 lines)
2. Updated table of contents in docs/toc.yml to include the new article in the Deploy > Azure section
3. Validated content using markdownlint to ensure consistency with repository standards

The documentation follows the Microsoft Writing Style Guide and matches the existing documentation patterns, including proper front matter, code examples, and cross-references to related articles.

Fixes #1766.

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Internal previews

📄 File	🔗 Preview link
docs/deployment/azure/azure-security-best-practices.md	docs/deployment/azure/azure-security-best-practices
docs/toc.yml	docs/toc