Reorganize directories (#3462)

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

docs/examples/basic_json__nullptr_t.cpp

@@ -0,0 +1,16 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // implicitly create a JSON null value
+    json j1;
+
+    // explicitly create a JSON null value
+    json j2(nullptr);
+
+    // serialize the JSON null value
+    std::cout << j1 << '\n' << j2 << '\n';
+}