[Feature Request] <list type="definition"> support

The **definition** list type is not a .NET standard, but widely supported since [NDoc](https://ndoc.sourceforge.net/usersguide.html) days, and I was actually surprised it was not working in DocFX.
The content is listed here for reference:
```xml

term
description


term
description

```

For the following documentation:
```csharp
///
/// Gets the numeric identifier of this code list.
///
/// A numerical value representing an ISO identifier of the code list.
///
///
/// By design, the ordinals are specified as follows:
///
///
///
/// Ordinal = 0
/// Represents an undefined or empty enumeration.
///
///
/// Ordinal >= 1
/// Represent the ISO defined enumerations of the code list.
///
///
///
public ushort Ordinal { get; }
```

The definition output is empty: