Add What's new in Aspire 13 article

[Copilot]

Adds comprehensive "What's new in Aspire 13" documentation based on milestone 8 issues, following the established format of previous release articles.

Content Coverage

AppHost & Orchestration

• Simplified AppHost SDK usage with implicit Aspire.Hosting.AppHost package references
• AddCSharpApp support for C# file-based apps
• Resource certificate trust customization

Polyglot Support

• Intuitive environment variable naming (PROJECTA_HTTP, PROJECTA_HTTPS) replacing service discovery patterns
• First-class Python support with templates and onboarding documentation

Dashboard

• OpenID Connect claims actions configuration for custom identity mapping

Security

• Encoded parameters for sensitive configuration data
• Breaking change: DefaultAzureCredential now uses only ManagedIdentityCredential on Azure Container Apps and App Service

Files Modified

• docs/whats-new/dotnet-aspire-13.md - New article (191 lines)
• docs/whats-new/toc.yml - Added Aspire 13 entry
• docs/whats-new/index.yml - Added Aspire 13 to release documentation and GitHub releases
• docs/toc.yml - Updated main TOC reference

Article includes AI-usage disclosure, follows Microsoft writing style guide, and provides migration guidance for breaking changes.

Original prompt

Please create a PR that targets the release-13 branch, and drafts up a "What's new in Aspire 13" article that follows the same format of the existing what's new content. Also, please use all issues in the https://github.com/dotnet/docs-aspire/milestone/8 to figure out the initial conent for this article.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

---

Internal previews

📄 File	🔗 Preview link
docs/toc.yml	docs/toc
docs/whats-new/dotnet-aspire-13.md	docs/whats-new/dotnet-aspire-13
docs/whats-new/index.yml	docs/whats-new/index
docs/whats-new/toc.yml	docs/whats-new/toc