Add manual doc on STL-compatible iterators

doc/snippets/Tutorial_std_sort.cpp

@@ -0,0 +1,4 @@
+Array4i v = Array4i::Random().abs();
+cout << "Here is the initial vector v:\n" << v.transpose() << "\n";
+std::sort(v.begin(), v.end());
+cout << "Here is the sorted vector v:\n" << v.transpose() << "\n";