Documentation for HTTP headers analyzer

[JamesNK]

New analyzer in dotnet/aspnetcore#44123

Should have matching docs.

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Associated WorkItem - 118228

[Rick-Anderson]

IHeaderDictionary has DIM properties for many commonly used headers: IHeaderDictionary.Keyed.cs

DIM?

[JamesNK]

Default interface implementation

[JamesNK]

This is .NET 8 btw

[tdykstra]

This was fixed in #27727
It's ASP0015