Reorganize directories (#3462)

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

docs/examples/iterator_wrapper.cpp

@@ -0,0 +1,29 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json j_object = {{"one", 1}, {"two", 2}};
+    json j_array = {1, 2, 4, 8, 16};
+
+    //////////////////////////////////////////////////////////////////////////
+    // The static function iterator_wrapper was deprecated in version 3.1.0
+    // and will be removed in version 4.0.0. Please replace all occurrences
+    // of iterator_wrapper(j) with j.items().
+    //////////////////////////////////////////////////////////////////////////
+
+    // example for an object
+    for (auto& x : json::iterator_wrapper(j_object))
+    {
+        std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
+    }
+
+    // example for an array
+    for (auto& x : json::iterator_wrapper(j_array))
+    {
+        std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
+    }
+}