Freshen string comparison code

[BillWagner]

• Fixes Update String.Split conceptual guide with more String.Split overloads #20123: Add an explain additional String.Split overloads.
• Fixes Is _Delegate equality_ definition unclear? #28006: Explain how delegate object instances are compared. Specifically, that the receiver is part of the equality comparison.
• Fixes String comparison article does not explain the correctness of using 'is' to compare strings. #42548: Provide details and links on how == is similar to the pattern matching is, and != is similar to the pattern matching is not.

Commit by commit history:

1. Move and update sample code.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/how-to/compare-strings.md	How to compare strings in C#
docs/csharp/how-to/concatenate-multiple-strings.md	How to concatenate multiple strings (C# Guide)
docs/csharp/how-to/modify-string-contents.md	"How to modify string contents"
docs/csharp/how-to/parse-strings-using-split.md	How to separate strings using String.Split in C#
docs/csharp/how-to/search-strings.md	docs/csharp/how-to/search-strings