Reorganize directories (#3462)

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

docs/examples/invalid_iterator.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    try
+    {
+        // calling iterator::key() on non-object iterator
+        json j = "string";
+        json::iterator it = j.begin();
+        auto k = it.key();
+    }
+    catch (json::invalid_iterator& e)
+    {
+        // output exception information
+        std::cout << "message: " << e.what() << '\n'
+                  << "exception id: " << e.id << std::endl;
+    }
+}