Reorganize directories (#3462)

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

docs/examples/parse__array__parser_callback_t.cpp

@@ -0,0 +1,30 @@
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // a JSON text
+    char text[] = R"(
+    {
+        "Image": {
+            "Width":  800,
+            "Height": 600,
+            "Title":  "View from 15th Floor",
+            "Thumbnail": {
+                "Url":    "http://www.example.com/image/481989943",
+                "Height": 125,
+                "Width":  100
+            },
+            "Animated" : false,
+            "IDs": [116, 943, 234, 38793]
+        }
+    }
+    )";
+
+    // parse and serialize JSON
+    json j_complete = json::parse(text);
+    std::cout << std::setw(4) << j_complete << "\n\n";
+}