System.Runtime.Serialization.DataContracts docs for .NET 10.0 APIs

# System.Runtime.Serialization.DataContracts docs for .NET 10.0 APIs

## Description

This issue tracks the porting of documentation for 7 APIs in the `System.Runtime.Serialization.DataContracts` namespace that were added in .NET 10.0.

## Instructions

Please follow the instructions in the main tracking issue (#120483) for documenting these APIs.

## Undocumented APIs

The following APIs need documentation:

### DataContract

- [M:System.Runtime.Serialization.DataContracts.DataContract.Equals(System.Object)](https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Runtime.Serialization.DataContracts/DataContract.xml)
- [M:System.Runtime.Serialization.DataContracts.DataContract.GetHashCode](https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Runtime.Serialization.DataContracts/DataContract.xml)

### XmlDataContract

- [P:System.Runtime.Serialization.DataContracts.XmlDataContract.ContractType](https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Runtime.Serialization.DataContracts/XmlDataContract.xml)
- [P:System.Runtime.Serialization.DataContracts.XmlDataContract.IsBuiltInDataContract](https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Runtime.Serialization.DataContracts/XmlDataContract.xml)
- [P:System.Runtime.Serialization.DataContracts.XmlDataContract.KnownDataContracts](https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Runtime.Serialization.DataContracts/XmlDataContract.xml)
- [P:System.Runtime.Serialization.DataContracts.XmlDataContract.TopLevelElementName](https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Runtime.Serialization.DataContracts/XmlDataContract.xml)
- [P:System.Runtime.Serialization.DataContracts.XmlDataContract.TopLevelElementNamespace](https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Runtime.Serialization.DataContracts/XmlDataContract.xml)

Total APIs to document: 7