Added callbacks for node errors and fifo overflow

Add new doc system which should/might deploy to pages.

examples/15_node_error_handler/main.cpp

@@ -44,6 +44,7 @@ int main() {
     auto proc = make_node<validate>();
     auto snk  = make_node<sink>   ();
 
+    // --8<-- [start:error_handler]
     // Return true  → skip this invocation, keep the node running.
     // Return false → stop the node (downstream drains then also stops).
     proc.set_error_handler([](std::string_view name, std::exception_ptr ep) {
@@ -53,6 +54,7 @@ int main() {
         }
         return true;
     });
+    // --8<-- [end:error_handler]
 
     Network net;
     net.add("source",    src)