add source for the seg tree walk

Author: lrvideckis

library/data_structures_[l,r)/seg_tree.hpp

@@ -1,5 +1,6 @@
 #pragma once
 //! https://github.com/kth-competitive-programming/kactl/blob/main/content/data-structures/SegmentTree.h
+//! https://codeforces.com/blog/entry/118682
 //! @code
 //!   tree st1(n, INT_MAX, ranges::min);
 //!   tree st2(n, INT_MAX, [&](int x, int y) -> int {