Add XML roundtripping documentation for carriage return entities

[Copilot]

The LINQ to XML whitespace preservation documentation was missing crucial information about XML roundtripping with carriage return entities. Users encountering XML with 
 entities would find that LINQ to XML's standard serialization doesn't preserve them, breaking roundtrip scenarios.

Problem

When XML contains carriage return entities like this:

<x xml:space="preserve">a&#xD;
b
c&#xD;</x>

Parsing with XDocument.Parse() correctly produces "a\r\nb\nc\r", but reserializing with LINQ to XML methods (even with SaveOptions.DisableFormatting) loses the carriage return entities:

<x xml:space="preserve">a
b
c</x>

Upon reparsing, the value becomes "a\nb\nc\n" - different from the original.

Solution

Added a new section "Roundtripping XML with carriage return entities" that:

• Explains the difference between LINQ to XML whitespace preservation and true XML roundtripping
• Demonstrates the issue with practical code examples
• Provides the solution using XmlWriter with NewLineHandling.Entitize
• Shows complete working code that preserves carriage return entities
• Links to relevant XmlWriter documentation

The documentation now guides users to use:

XmlWriterSettings settings = new XmlWriterSettings
{
    NewLineHandling = NewLineHandling.Entitize,
    OmitXmlDeclaration = true
};

This enhancement provides the missing guidance requested in the original issue while maintaining consistency with existing documentation style and following the Microsoft Writing Style Guide.

Fixes #9680.

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