Added callbacks for node errors and fifo overflow

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

examples/04_storage_policy/main.cpp

@@ -34,14 +34,14 @@ struct Tag {
     int value = 0;
 };
 
-// [snippet: storage_policy_spec]
+// --8<-- [start:storage_policy_spec]
 // Override: store Tag by value despite being a struct
 // (it's trivially copyable and small — this just makes the policy explicit)
 template<>
 struct kpn::channel_storage_policy<Tag> {
     static constexpr bool by_value = true;
 };
-// [/snippet: storage_policy_spec]
+// --8<-- [end:storage_policy_spec]
 
 // ── Node functions ────────────────────────────────────────────────────────────