Reorganize directories (#3462)

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

docs/examples/json_pointer__empty.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // different JSON Pointers
+    json::json_pointer ptr0;
+    json::json_pointer ptr1("");
+    json::json_pointer ptr2("/foo");
+    json::json_pointer ptr3("/foo/0");
+
+    // call empty()
+    std::cout << std::boolalpha
+              << ptr0 << ": " << ptr0.empty() << '\n'
+              << ptr1 << ": " << ptr1.empty() << '\n'
+              << ptr2 << ": " << ptr2.empty() << '\n'
+              << ptr3 << ": " << ptr3.empty() << std::endl;
+}