Reorganize directories (#3462)

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

docs/examples/json_pointer__back.cpp

@@ -0,0 +1,15 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // different JSON Pointers
+    json::json_pointer ptr1("/foo");
+    json::json_pointer ptr2("/foo/0");
+
+    // call empty()
+    std::cout << "last reference token of " << ptr1 << " is " << ptr1.back() << '\n'
+              << "last reference token of " << ptr2 << " is " << ptr2.back() << std::endl;
+}