docs(engine): document and cleanup Hello Cubos sample

docs/pages/3_examples/2_engine/main.md

@@ -5,6 +5,7 @@
 The following examples have fully documented tutorials on how to use the
 multiple plugins of the engine:
 
+- @subpage examples-engine-hello-cubos - @copybrief examples-engine-hello-cubos
 - @subpage examples-engine-settings - @copybrief examples-engine-settings
 - @subpage examples-engine-renderer - @copybrief examples-engine-renderer
 - @subpage examples-engine-scene - @copybrief examples-engine-scene

engine/samples/CMakeLists.txt

@@ -17,6 +17,7 @@ macro(make_sample)
 
     # Get the source files
     set(sources "${CMAKE_CURRENT_SOURCE_DIR}/${MAKE_SAMPLE_DIR}/main.cpp")
+
     foreach(source IN LISTS MAKE_SAMPLE_SOURCES)
         list(APPEND sources "${CMAKE_CURRENT_SOURCE_DIR}/${MAKE_SAMPLE_DIR}/${source}")
     endforeach()
@@ -36,9 +37,9 @@ macro(make_sample)
 endmacro()
 
 # Add samples
+make_sample(DIR "hello-cubos")
 make_sample(DIR "settings")
 make_sample(DIR "events")
-make_sample(DIR "systems")
 make_sample(DIR "input" ASSETS)
 make_sample(DIR "assets/json" ASSETS)
 make_sample(DIR "assets/bridge" ASSETS)

engine/samples/hello-cubos/main.cpp

@@ -0,0 +1,45 @@
+#include <cubos/engine/cubos.hpp>
+
+/// [Hello Cubos]
+void sayHelloCubos()
+{
+    CUBOS_INFO("Hello CUBOS");
+}
+/// [Hello Cubos]
+
+/// [Hello World]
+void sayHello()
+{
+    CUBOS_INFO("Hello");
+}
+
+void sayWorld()
+{
+    CUBOS_INFO("World");
+}
+/// [Hello World]
+
+int main()
+{
+    /// [Engine]
+    cubos::engine::Cubos cubos;
+    /// [Engine]
+
+    /// [Tags]
+    cubos.tag("exampleTagA").before("exampleTagB");
+    cubos.startupTag("exampleTagB");
+    /// [Tags]
+
+    /// [Set Startup]
+    cubos.startupSystem(sayHelloCubos);
+    /// [Set Startup]
+
+    /// [Set Systems]
+    cubos.system(sayHello);
+    cubos.system(sayWorld);
+    /// [Set Systems]
+
+    /// [Run]
+    cubos.run();
+    /// [Run]
+}

engine/samples/hello-cubos/page.md

@@ -0,0 +1,50 @@
+# Settings {#examples-engine-hello-cubos}
+
+@brief Using @ref cubos::engine::Cubos to create a simple program.
+
+This example shows the basics of how @ref cubos::engine::Cubos is used, by making a simple "Hello World" program.
+
+The first thing we'll need to do is get the engine itself.
+
+@snippet hello-cubos/main.cpp Engine
+
+The @ref cubos::engine::Cubos class represents the engine.
+We'll need it to add functionality to our program.
+
+Let's start by defining what functionality we want to add.
+
+@snippet hello-cubos/main.cpp Hello Cubos
+
+This function simply prints `Hello CUBOS` to the console.
+It uses one of CUBOS.'s logging macros.
+You can find more about them @ref core\include\cubos\core\log.hpp "here".
+However, this function is not currently called, so we'll need to tell CUBOS. that we want it to run.
+
+@snippet hello-cubos/main.cpp Set Startup
+
+Startup systems run only once when the engine is loaded.
+Now let's make things more interesting.
+Let's print `Hello World` every single frame.
+And let's also split it over two different systems.
+
+@snippet hello-cubos/main.cpp Hello World
+
+Instead of using `startupSystem`, we'll use @ref cubos::engine::Cubos::system .
+This means the systems will be called every cycle, instead of just once at startup.
+However, we can't just do as we did for `sayHelloCubos` and call it a day.
+We want `sayHello` to come before `sayWorld`, so we'll have to explicitly tell that to the engine, or else we risk having them in the wrong order.
+To do that we use tags.
+Let's create two tags, one for each system.
+
+@snippet hello-cubos/main.cpp Tags
+
+@ref cubos::engine::Cubos::tag creates a new tag, and @ref cubos::engine::TagBuilder::before makes any systems tagged with it come before systems tagged with the one given as parameter.
+There's also a @ref cubos::engine::TagBuilder::after that has the inverse effect.
+Now all we have to do is to assign these tags to our systems.
+@note If we wanted to give these tags to a system running on startup, we'd have to use @ref cubos::engine::Cubos::startupTag instead.
+
+@snippet hello-cubos/main.cpp Set Systems
+
+With everything properly set up, all that remains is to run the engine.
+
+@snippet hello-cubos/main.cpp Run