Clarify differences between AddDockerfile and WithDockerfile methods

[Copilot]

The "Add Dockerfiles to your .NET app model" documentation previously mentioned both AddDockerfile() and WithDockerfile() methods without explaining their differences, when to use each, or whether they create Dockerfile files.

This PR adds clear explanations that address the confusion:

Key improvements:

1. Clear definitions in introduction: Added explanations that AddDockerfile creates new container resources while WithDockerfile customizes existing ones

2. New "When to use" section: Provides specific guidance on choosing the appropriate method:

   • Use AddDockerfile for custom containerized services
   • Use WithDockerfile to customize existing .NET Aspire components

3. Clarifies Dockerfile expectations: Explicitly states that neither method creates Dockerfile files - both expect existing Dockerfiles in the specified context path

4. Enhanced examples: Improved the PostgreSQL example with comments explaining the customization behavior

Before:

With .NET Aspire it's possible to specify a Dockerfile... using either the AddDockerfile or WithDockerfile extension methods.

After:

These two methods serve different purposes:
- AddDockerfile: Creates a new container resource from an existing Dockerfile...
- WithDockerfile: Customizes an existing container resource...

Both methods expect an existing Dockerfile in the specified context path—neither method creates a Dockerfile for you.

The changes are minimal (28 lines added, 2 modified) and maintain the existing document structure while directly addressing the user's confusion.

Fixes #1824.

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