Enhance EF Core migrations documentation with troubleshooting and multiple database guidance

[Copilot]

This PR enhances the EF Core migrations documentation to address common issues developers face when working with Entity Framework migrations in .NET Aspire projects.

Key Enhancements

1. Package Manager Console Instructions

Added comprehensive guidance for using Visual Studio's Package Manager Console as an alternative to CLI commands:

• Step-by-step instructions for Package Manager Console setup
• Clear guidance on startup project vs default project configuration
• Proper project selection requirements

2. Troubleshooting Section

Added detailed troubleshooting for the most common migration issues:

"No database provider has been configured" error:

• Explanation of why this occurs in .NET Aspire projects
• Step-by-step solution using temporary connection strings
• Clear guidance on connection string naming to match app host configuration

Multiple databases scenarios:

• Two approaches: separate migration services vs single service with multiple contexts
• Code examples for both approaches
• Best practices for managing multiple database schemas

Startup project configuration:

• Clear distinction between CLI and Package Manager Console requirements
• Guidance on which projects to select for different scenarios

3. Enhanced Clarity

• Better explanation of where migrations should be created
• Clarification that migration services run before other services start
• Improved project structure guidance

Problem Solved

The original documentation existed but lacked practical troubleshooting guidance that developers needed when encountering common migration issues in .NET Aspire projects. This enhancement provides clear, actionable solutions to the specific problems mentioned in the GitHub issue.

Example of Enhanced Content

Before: Basic CLI instructions only
After: Both CLI and Package Manager Console instructions with troubleshooting

{
  "ConnectionStrings": {
    "ticketdb": "Server=(localdb)\\mssqllocaldb;Database=TicketDb;Trusted_Connection=true"
  }
}

The enhanced documentation now covers edge cases and provides solutions that work in real-world development scenarios.

Fixes #1664.

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Internal previews

📄 File	🔗 Preview link
docs/database/ef-core-migrations.md	docs/database/ef-core-migrations

[IEvangelist]

@alistairmatthews - might you have a read through of this, since you've done a lot recently with migrations, I'd be curious what you think of these changes.

[alistairmatthews]

I've left a few comments. I'm going to test the Multiple database scenario code next week. Ran out of time today.

[adegeo]

Approving to unblock