Added callbacks for node errors and fifo overflow

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

examples/03_multi_output/main.cpp

@@ -33,7 +33,7 @@ static std::string generate() {
     return pairs[gen_index++ % 5];
 }
 
-// [snippet: multi_output_fn]
+// --8<-- [start:multi_output_fn]
 // Multi-output: returns (key, value) as a tuple — KPN++ routes each element
 // to its own output port automatically.
 static std::tuple<std::string, std::string> parse(std::string kv) {
@@ -41,7 +41,7 @@ static std::tuple<std::string, std::string> parse(std::string kv) {
     if (sep == std::string::npos) return {kv, ""};
     return {kv.substr(0, sep), kv.substr(sep + 1)};
 }
-// [/snippet: multi_output_fn]
+// --8<-- [end:multi_output_fn]
 
 static void print_key(std::string key) {
     std::cout << "KEY   → " << key << '\n';
@@ -56,7 +56,7 @@ static void print_value(std::string value) {
 int main() {
     using namespace kpn;
 
-    // [snippet: fanout_network]
+    // --8<-- [start:fanout_network]
     auto gen  = make_node<generate>(out<"kv">{},          4);
     auto par  = make_node<parse>   (in<"kv">{},  out<"key", "value">{}, 4);
     auto keys = make_node<print_key>  (in<"key">{},   4);
@@ -75,5 +75,5 @@ int main() {
     net.start();
     std::this_thread::sleep_for(std::chrono::milliseconds(600));
     net.stop();
-    // [/snippet: fanout_network]
+    // --8<-- [end:fanout_network]
 }