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.
2. Update equality operators to enhance the description of delegate equality. This needs to ensure that the receiver instances are the same object if the target method is an instance method.
3. Compare == with is for equality comparisons with constants.
4. Add more overload examples and a summary to the String.Split article.
5. Final edit pass.

---

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
docs/csharp/language-reference/operators/equality-operators.md	Equality operators - test if two objects are equal or not
docs/csharp/language-reference/operators/is.md	"The is operator - Match an expression against a type or constant pattern"
docs/csharp/language-reference/operators/patterns.md	Pattern matching - the is and switch expressions, and operators and, or, and not in patterns

[CamSoper]

LGTM. Approving with a couple comments.