Added callbacks for node errors and fifo overflow

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

examples/01_hello_pipeline/main.cpp

@@ -7,20 +7,20 @@
 //
 //  [produce] --int--> [double_it] --int--> [print_it]
 
-// [snippet: basic_node_fns]
+// --8<-- [start:basic_node_fns]
 static int  produce()        { return 42; }
 static int  double_it(int x) { return x * 2; }
 static void print_it(int x)  { std::cout << "result: " << x << '\n'; }
-// [/snippet: basic_node_fns]
+// --8<-- [end:basic_node_fns]
 
 int main() {
     using namespace kpn;
 
-    // [snippet: index_only_nodes]
+    // --8<-- [start:index_only_nodes]
     auto src  = make_node<produce>(5);
     auto dbl  = make_node<double_it>(5);
     auto sink = make_node<print_it>(5);
-    // [/snippet: index_only_nodes]
+    // --8<-- [end:index_only_nodes]
 
     // Wire channels
     auto& dbl_in  = dbl.input_channel<0>();
@@ -28,7 +28,7 @@ int main() {
     src.set_output_channel<0>(&dbl_in);
     dbl.set_output_channel<0>(&sink_in);
 
-    // [snippet: network_build]
+    // --8<-- [start:network_build]
     Network net;
     net.add("src",  src)
        .add("dbl",  dbl)
@@ -40,5 +40,5 @@ int main() {
     net.start();
     std::this_thread::sleep_for(std::chrono::milliseconds(100));
     net.stop();
-    // [/snippet: network_build]
+    // --8<-- [end:network_build]
 }