docs(imgui): introduce ImGuiContextHolder in sample

engine/samples/imgui/main.cpp

@@ -96,6 +96,12 @@ int main(int argc, char** argv)
     cubos.plugin(imguiPlugin);
     /// [Adding the plugin]
 
+    /// [ImGui initialization]
+    cubos.startupSystem("set ImGui context").after(imguiInitTag).call([](ImGuiContextHolder& holder) {
+        ImGui::SetCurrentContext(holder.context);
+    });
+    /// [ImGui initialization]
+
     /// [ImGui Demo]
     cubos.system("show ImGui demo").tagged(imguiTag).call([]() {
         ImGui::Begin("Dear ImGui + Cubos");

engine/samples/imgui/page.md

@@ -16,7 +16,13 @@ Then, make sure to add the plugin.
 
 @snippet imgui/main.cpp Adding the plugin
 
-Once the ImGui plugin is added, you can create systems to display ImGui windows and widgets. Here's a system which opens an ImGui window, and its demo.
+When you're using ImGui directly in an application or a library, you must also make sure to initialize the ImGui context on it.
+To do so, you can add a startup system just like the one below:
+
+@snippet imgui/main.cpp ImGui initialization
+
+You can read more about this in the documentation of @ref cubos::engine::ImGuiContextHolder.
+Now, you can create systems to display ImGui windows and widgets. Here's a system which opens an ImGui window, and its demo.
 
 @snippet imgui/main.cpp ImGui Demo