Enhance examples (#17)

* enhance examples

* doc enhancement

* mv pool to stack

* missing ; and doc update

fix readme example image link

examples/custom_0/custom_0.cpp

@@ -0,0 +1,53 @@
+#include <memory>
+#include <string>
+
+#include "DefaultApp.hpp"
+#include "resources/resource.hpp"
+#include "utilities/ILog.hpp"
+#include "utilities/SpdLogger.hpp"
+
+using namespace pixelarium;
+using namespace std;
+using Log = utils::log::ILog;
+using Pool = resources::ImageResourcePool;
+
+// setup a logger
+#ifdef _WIN32
+unique_ptr<Log> logger{
+    make_unique<utils::log::SpdLogger>(string(getenv("APPDATA")) + "/pixelarium/simple_app.log", "default")};
+#else
+unique_ptr<Log> logger{
+    make_unique<utils::log::SpdLogger>(string(getenv("HOME")) + "/.cache/pixelarium/simple_app.log", "default")};
+#endif
+
+// create a custom app inheriting from the library's default app
+class MyApp : public application::DefaultApp
+{
+   public:
+    MyApp(const Log& log, Pool& pool) : application::DefaultApp(log, pool) {}
+
+    // override some of the defaults member functions
+    void Run() override;
+    void MenuBarOptionsColumn1() override {};
+    void MenuBarOptionsColumn2() override {};
+};
+
+int main()
+{
+    // some initial log message
+    logger->Info(std::format("{}: Starting Application {}", __FUNCTION__, "Pixelarium"));
+
+    // instantiate an image pool for the application
+    resources::ImageResourcePool image_pool;
+
+    // create a custom application, inject its dependencies and start it
+    auto app{MyApp(*logger, image_pool)};
+
+    app.Start();
+}
+
+void MyApp::Run()
+{
+    this->gallery_.RenderGallery();
+    this->gallery_.RenderImages();
+}