Add reference and conceptual content for ref struct allowed in interfaces

[BillWagner]

Contributes to #41206 (A following PR will add updated compiler diagnostics.)

Fixes other issues in the articles that were modified by this PR:

Fixes #33689: Update description of AssemblyFlagsAttribute.
Fixes #41392: Add allowed access modifiers on interface declarations and interface member declarations.
Fixes #39364: Add a definition of "base interface".
Fixes #40268: Update multiple constraints sample
Fixes #41761: Define "method group"

Commit details:

1. Publish speclet.
2. Add language reference for new syntax allow ref struct, and rules for ref struct types implementing an interface.
3. Add note on interfaces that default interface members must be implemented by ref struct types that implement that interface.
4. Edit pass and fix existing issues.
5. Update the what's new page.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/attributes/general.md	Miscellaneous attributes interpreted by the C# compiler
docs/csharp/language-reference/attributes/global.md	"Attributes interpreted by the compiler: Global attributes"
docs/csharp/language-reference/builtin-types/ref-struct.md	ref structure types (C# reference)
docs/csharp/language-reference/keywords/index.md	C# Keywords
docs/csharp/language-reference/keywords/interface.md	:::no-loc text="interface"::: (C# Reference)
docs/csharp/language-reference/keywords/where-generic-type-constraint.md	where (generic type constraint) (C# Reference)
docs/csharp/programming-guide/generics/constraints-on-type-parameters.md	Constraints on type parameters (C# Programming Guide)
docs/csharp/whats-new/csharp-13.md	docs/csharp/whats-new/csharp-13

[BillWagner]

ping @adegeo for review.

[adegeo]

LGTM! Just a few minor things to consider.