Add comprehensive Virtual Threads documentation

This commit adds a new documentation page covering virtual threads support
in Apache Camel, including:

- Introduction to virtual threads and why they matter for integration
- How to enable virtual threads globally in Camel
- Components with virtual thread support (SEDA, Jetty, Platform HTTP, etc.)
- SEDA deep dive with two execution models comparison (traditional vs
  virtualThreadPerTask) including a Mermaid diagram
- Backpressure and flow control mechanisms
- Context propagation with ContextValue (ThreadLocal vs ScopedValue)
- Best practices and performance considerations
- Complete code examples for common use cases

The article is added to the navigation under Architecture, after
Threading Model.

Author: gnodet

docs/user-manual/modules/ROOT/nav.adoc

@@ -92,6 +92,7 @@
 ** xref:template-engines.adoc[Template Engines]
 ** xref:transformer.adoc[Transformer]
 ** xref:threading-model.adoc[Threading Model]
+** xref:virtual-threads.adoc[Virtual Threads]
 ** xref:tracer.adoc[Tracer]
 ** xref:type-converter.adoc[Type Converter]
 ** xref:uris.adoc[URIs]