Add examples of a spread element to the "Collection initializers" article

[BartoszKlonowski]

This pull request closes #44032

It presents the spread element examples by:

• Creating a list out of other lists
• Creating a list out of other lists and single element in between, to show that those syntax can be mixed
• Using the spread element syntax to create a full list of all cats that is displayed at the very end of the collection full example

Each example is described with brief intro as other examples in that section.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/programming-guide/classes-and-structs/object-and-collection-initializers.md	Object and Collection Initializers (C# Programming Guide)

[BillWagner]

Thanks again @BartoszKlonowski

This looks great, and I'll now.