Add comprehensive PostgreSQL EF Core migrations tutorial for .NET Aspire

[Copilot]

This PR addresses the critical gap in .NET Aspire documentation by providing a comprehensive tutorial for PostgreSQL Entity Framework Core migrations - something that has been repeatedly requested by the community but was missing from the documentation.

What this adds

New Tutorial: postgresql-ef-core-migrations-tutorial.md - A complete 314-line, step-by-step guide that covers:

• Setting up .NET Aspire projects with PostgreSQL and Entity Framework Core from scratch
• Creating and configuring EF Core models and database contexts
• Actually creating migrations (not just applying existing ones) - the core missing piece
• Building and configuring migration services for automatic schema deployment
• PostgreSQL-specific patterns and troubleshooting scenarios
• Complete working application with data persistence

Complete Working Code Examples: All tutorial code is fully functional with 9 supporting files including:

• Entity models with PostgreSQL-optimized configurations
• DbContext setup for PostgreSQL
• Migration service implementation for automatic schema updates
• App host orchestration with proper service dependencies
• Complete Blazor UI for testing the end-to-end workflow

Why this was needed

The existing documentation had:

• A general EF Core migrations guide focused on SQL Server
• PostgreSQL integration docs without migration coverage
• No end-to-end tutorial showing how to create migrations (only apply existing ones)

This left developers unable to figure out how to:

• Generate PostgreSQL migrations in .NET Aspire projects
• Set up migration services for PostgreSQL
• Handle PostgreSQL-specific differences from SQL Server examples

Key differentiators from existing content

Unlike the existing SQL Server tutorial, this focuses specifically on:

• PostgreSQL container orchestration patterns
• PostgreSQL-specific EF Core configuration
• Handling connection string challenges with containerized PostgreSQL
• PostgreSQL naming conventions and data type considerations
• Common PostgreSQL + .NET Aspire troubleshooting scenarios

The tutorial provides the "real end-to-end tutorial that shows how to actually create migrations" that was explicitly requested in the issue, with working code that developers can follow from start to finish.

Fixes #1581.

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Internal previews

📄 File	🔗 Preview link
docs/database/postgresql-ef-core-migrations-tutorial.md	docs/database/postgresql-ef-core-migrations-tutorial
docs/toc.yml	docs/toc