Reorganize directories (#3462)

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

docs/examples/out_of_range.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    try
+    {
+        // calling at() for an invalid index
+        json j = {1, 2, 3, 4};
+        j.at(4) = 10;
+    }
+    catch (json::out_of_range& e)
+    {
+        // output exception information
+        std::cout << "message: " << e.what() << '\n'
+                  << "exception id: " << e.id << std::endl;
+    }
+}