update summary.md

Author: thanhnguyen-aws

doc/src/SUMMARY.md

@@ -29,3 +29,13 @@
   - [13: Safety of `CStr`](./challenges/0013-cstr.md)
   - [14: Safety of Primitive Conversions](./challenges/0014-convert-num.md)
   - [15: Contracts and Tests for SIMD Intrinsics](./challenges/0015-intrinsics-simd.md)
+  - [16: Verify the safety of Iterator functions](./challenges/0016-iter.md)
+  - [17: Verify the safety of slice functions](./challenges/0017-slice.md)
+  - [18: Verify the safety of slice iter functions - part 1](./challenges/0018-slice-iter-pt1.md)
+  - [19: Verify the safety of slice iter functions - part 2](./challenges/0019-slice-iter-pt2.md)
+  - [20: Verify the safety of char-related functions in str::pattern](./challenges/0020-str-pattern-pt1.md)
+  - [21: Verify the safety of substring-related functions in str::pattern](./challenges/0021-str-pattern-pt2.md)
+  - [22: Verify the safety of str iter functions](./challenges/0022-str-iter.md)
+  - [23: Verify the safety of Vec functions part 1](./challenges/0023-vec-pt1.md)
+  - [24: Verify the safety of Vec functions part 2](./challenges/0024-vec-pt2.md)
+  - [26: Verify the safety of `VecDeque` functions](./challenges/0025-vecdeque.md)