Reorganize directories (#3462)

* 🚚 move files
* 🚚 rename doc folder to docs
* 🚚 rename test folder to tests

docs/examples/push_back__initializer_list.cpp

@@ -0,0 +1,27 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json object = {{"one", 1}, {"two", 2}};
+    json null;
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << null << '\n';
+
+    // add values:
+    object.push_back({"three", 3});  // object is extended
+    object += {"four", 4};           // object is extended
+    null.push_back({"five", 5});     // null is converted to array
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << null << '\n';
+
+    // would throw:
+    //object.push_back({1, 2, 3});
+}