Reorganize directories (#3462)

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

docs/examples/operator_serialize.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <iomanip>
+#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};
+
+    // serialize without indentation
+    std::cout << j_object << "\n\n";
+    std::cout << j_array << "\n\n";
+
+    // serialize with indentation
+    std::cout << std::setw(4) << j_object << "\n\n";
+    std::cout << std::setw(2) << j_array << "\n\n";
+    std::cout << std::setw(1) << std::setfill('\t') << j_object << "\n\n";
+}