📝 added documentation

doc/examples/json_pointer__push_back.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create empty JSON Pointer
+    json::json_pointer ptr;
+    std::cout << ptr << '\n';
+
+    // call push_back()
+    ptr.push_back("foo");
+    std::cout << ptr << '\n';
+
+    ptr.push_back("0");
+    std::cout << ptr << '\n';
+
+    ptr.push_back("bar");
+    std::cout << ptr << '\n';
+}