feat(doc extractor): support for cref, paramref and langword xml doc elements

[jfonseca-aneo]

Motivation

C# XML documentation commonly references other types and members using , parameters
using , and language keywords using . These are first-class
constructs in the doc comment syntax, but the extractor was treating them as unknown nodes and silently
dropping their content which might leave gaps in the generated markdown.

Summary

This PR introduces the following changes:

• Add handling for XmlEmptyElementSyntax nodes in RenderXmlNodes, which Roslyn produces for
  self-closing XML doc tags — previously silently ignored
• Render and as inline code using the referenced symbol name
• Render as inline code (e.g. null, true)
• Render and as inline code
• Handle the non-self-closing form display text, preferring the explicit display
  text over the cref