Rework IEquatable<T> example

[antonfirsov]

• Simplify & modernize the existing obscure and long code
  • Remove validation (it's irrelevant)
  • Rename SSN to NationalId (SSN doesn't mean much to non-US folks...)
  • Make the Contains usage sample simpler
• Add project files, remove unused source files
• Copy the IEquatable.Equals sample to the IEquatable base docs so it's more accessible

Fixes #11055

[learn-build-service-prod]

Learn Build status updates of commit b240967:

✅ Validation status: passed

File	Status	Preview URL	Details
snippets/csharp/System/IEquatableT/Equals/Equals.csproj	✅Succeeded
snippets/csharp/System/IEquatableT/Equals/EqualsEx1.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System/IEquatableT/Equals/EqualsEx2.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System/IEquatableT/Equals/EqualsExample.cs	✅Succeeded	View
snippets/csharp/System/IEquatableT/Equals/Snippet12.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System/IEquatableT/Equals/Equals.fsproj	✅Succeeded
snippets/fsharp/System/IEquatableT/Equals/EqualsEx2.fs	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System/IEquatableT/Equals/EqualsExample.fs	✅Succeeded	View
snippets/fsharp/System/IEquatableT/Equals/fs.fsproj	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System/IEquatableT/Equals/Snippet12.fs	✅Succeeded	n/a (file deleted or renamed)
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/Equals.vbproj	✅Succeeded
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/EqualsEx2.vb	✅Succeeded	n/a (file deleted or renamed)
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/EqualsExample.vb	✅Succeeded	View
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/Snippet12.vb	✅Succeeded	n/a (file deleted or renamed)
xml/System/IEquatable`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

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[learn-build-service-prod]

Learn Build status updates of commit d042eeb:

✅ Validation status: passed

File	Status	Preview URL	Details
snippets/csharp/System/IEquatableT/Equals/Equals.csproj	✅Succeeded
snippets/csharp/System/IEquatableT/Equals/EqualsEx1.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System/IEquatableT/Equals/EqualsEx2.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/csharp/System/IEquatableT/Equals/EqualsExample.cs	✅Succeeded	View
snippets/csharp/System/IEquatableT/Equals/Snippet12.cs	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System/IEquatableT/Equals/Equals.fsproj	✅Succeeded
snippets/fsharp/System/IEquatableT/Equals/EqualsEx2.fs	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System/IEquatableT/Equals/EqualsExample.fs	✅Succeeded	View
snippets/fsharp/System/IEquatableT/Equals/fs.fsproj	✅Succeeded	n/a (file deleted or renamed)
snippets/fsharp/System/IEquatableT/Equals/Snippet12.fs	✅Succeeded	n/a (file deleted or renamed)
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/Equals.vbproj	✅Succeeded
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/EqualsEx2.vb	✅Succeeded	n/a (file deleted or renamed)
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/EqualsExample.vb	✅Succeeded	View
snippets/visualbasic/VS_Snippets_CLR_System/system.GenericIEquatable.Equals/vb/Snippet12.vb	✅Succeeded	n/a (file deleted or renamed)
xml/System/IEquatable`1.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel