feat: add ruby code block - chapter backtracking

docs/chapter_backtracking/backtracking_algorithm.md

@@ -406,7 +406,27 @@
 === "Ruby"
 
     ```ruby title=""
+    ### 回溯算法框架 ###
+    def backtrack(state, choices, res)
+        # 判断是否为解
+        if is_solution?(state)
+            # 记录解
+            record_solution(state, res)
+            return
+        end
 
+        # 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)
+            end
+        end
+    end
     ```
 
 === "Zig"