Added callbacks for node errors and fifo overflow

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

examples/02_named_ports/main.cpp

@@ -54,7 +54,7 @@ static void report(int count, std::vector<std::string> words) {
 int main() {
     using namespace kpn;
 
-    // [snippet: named_port_creation]
+    // --8<-- [start:named_port_creation]
     // tokenise: no inputs, one named output "words"
     auto tok = make_node<tokenise>(out<"words">{}, 4);
 
@@ -63,9 +63,9 @@ int main() {
 
     // report: two named inputs
     auto snk = make_node<report>(in<"count", "words">{}, 4);
-    // [/snippet: named_port_creation]
+    // --8<-- [end:named_port_creation]
 
-    // [snippet: named_port_network]
+    // --8<-- [start:named_port_network]
     Network net;
     net.add("tok", tok)
        .add("cnt", cnt)
@@ -78,5 +78,5 @@ int main() {
     net.start();
     std::this_thread::sleep_for(std::chrono::milliseconds(500));
     net.stop();
-    // [/snippet: named_port_network]
+    // --8<-- [end:named_port_network]
 }