feat: add rust docs (#815)

* feat: add rust docs

* Import std types for built_in_hash doc

docs/chapter_backtracking/backtracking_algorithm.md

@@ -532,7 +532,27 @@
 === "Rust"
 
     ```rust title=""
-
+    /* 回溯算法框架 */
+    fn backtrack(state: &mut State, choices: &Vec<Choice>, res: &mut Vec<State>) {
+        // 判断是否为解
+        if is_solution(state) {
+            // 记录解
+            record_solution(state, res);
+            // 停止继续搜索
+            return;
+        }
+        // 遍历所有选择
+        for choice in choices {
+            // 剪枝：判断选择是否合法
+            if is_valid(state, choice) {
+                // 尝试：做出选择，更新状态
+                make_choice(state, choice);
+                backtrack(state, choices, res);
+                // 回退：撤销选择，恢复到之前的状态
+                undo_choice(state, choice);
+            }
+        }
+    }
     ```
 
 === "C"