Reorganize directories (#3462)

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

docs/examples/flatten.cpp

@@ -0,0 +1,32 @@
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON value
+    json j =
+    {
+        {"pi", 3.141},
+        {"happy", true},
+        {"name", "Niels"},
+        {"nothing", nullptr},
+        {
+            "answer", {
+                {"everything", 42}
+            }
+        },
+        {"list", {1, 0, 2}},
+        {
+            "object", {
+                {"currency", "USD"},
+                {"value", 42.99}
+            }
+        }
+    };
+
+    // call flatten()
+    std::cout << std::setw(4) << j.flatten() << '\n';
+}