Documentation change (#3672)

Co-authored-by: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>

docs/examples/nlohmann_json_namespace.cpp

@@ -0,0 +1,14 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+// possible use case: use NLOHMANN_JSON_NAMESPACE instead of nlohmann
+using json = NLOHMANN_JSON_NAMESPACE::json;
+
+// macro needed to output the NLOHMANN_JSON_NAMESPACE as string literal
+#define Q(x) #x
+#define QUOTE(x) Q(x)
+
+int main()
+{
+    std::cout << QUOTE(NLOHMANN_JSON_NAMESPACE) << std::endl;
+}