Reorganize directories (#3462)

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

docs/examples/operator_literal_json.cpp

@@ -0,0 +1,12 @@
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    json j = R"( {"hello": "world", "answer": 42} )"_json;
+
+    std::cout << std::setw(2) << j << '\n';
+}