Added callbacks for node errors and fifo overflow

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

examples/05_error_handling/main.cpp

@@ -45,7 +45,7 @@ int main() {
 
     Network net;
 
-    // [snippet: diagnostics_handler]
+    // --8<-- [start:diagnostics_handler]
     // Custom diagnostics handler — fires on the watchdog interval.
     // Print a concise one-liner rather than the full table.
     net.set_diagnostics_handler([](const std::vector<NodeSnapshot>& nodes,
@@ -58,7 +58,7 @@ int main() {
                       << "overflows=" << c.overflows;
         std::cout << '\n';
     });
-    // [/snippet: diagnostics_handler]
+    // --8<-- [end:diagnostics_handler]
 
     net.set_watchdog_interval(std::chrono::milliseconds(200));