Clean up and document project files (#4560)

docs/mkdocs/docs/examples/emplace_back.cpp

@@ -0,0 +1,24 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json array = {1, 2, 3, 4, 5};
+    json null;
+
+    // print values
+    std::cout << array << '\n';
+    std::cout << null << '\n';
+
+    // add values
+    array.emplace_back(6);
+    null.emplace_back("first");
+    null.emplace_back(3, "second");
+
+    // print values
+    std::cout << array << '\n';
+    std::cout << null << '\n';
+}