docs: add Japanese translate documents (#1812)

* docs: add Japanese documents (`ja/docs`)

* docs: add Japanese documents (`ja/codes`)

* docs: add Japanese documents

* Remove pythontutor blocks in ja/

* Add an empty at the end of each markdown file.

* Add the missing figures (use the English version temporarily).

* Add index.md for Japanese version.

* Add index.html for Japanese version.

* Add missing index.assets

* Fix backtracking_algorithm.md for Japanese version.

* Add avatar_eltociear.jpg. Fix image links on the Japanese landing page.

* Add the Japanese banner.

---------

Co-authored-by: krahets <krahets@163.com>

ja/codes/java/chapter_searching/two_sum.java

@@ -0,0 +1,53 @@
+/**
+ * File: two_sum.java
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_searching;
+
+import java.util.*;
+
+public class two_sum {
+    /* 方法一: 暴力列挙 */
+    static int[] twoSumBruteForce(int[] nums, int target) {
+        int size = nums.length;
+        // 二重ループ、時間計算量は O(n^2)
+        for (int i = 0; i < size - 1; i++) {
+            for (int j = i + 1; j < size; j++) {
+                if (nums[i] + nums[j] == target)
+                    return new int[] { i, j };
+            }
+        }
+        return new int[0];
+    }
+
+    /* 方法二: 補助ハッシュテーブル */
+    static int[] twoSumHashTable(int[] nums, int target) {
+        int size = nums.length;
+        // 補助ハッシュテーブル、空間計算量は O(n)
+        Map<Integer, Integer> dic = new HashMap<>();
+        // 単一層ループ、時間計算量は O(n)
+        for (int i = 0; i < size; i++) {
+            if (dic.containsKey(target - nums[i])) {
+                return new int[] { dic.get(target - nums[i]), i };
+            }
+            dic.put(nums[i], i);
+        }
+        return new int[0];
+    }
+
+    public static void main(String[] args) {
+        // ======= テストケース =======
+        int[] nums = { 2, 7, 11, 15 };
+        int target = 13;
+
+        // ====== ドライバーコード ======
+        // 方法一
+        int[] res = twoSumBruteForce(nums, target);
+        System.out.println("方法一 res = " + Arrays.toString(res));
+        // 方法二
+        res = twoSumHashTable(nums, target);
+        System.out.println("方法二 res = " + Arrays.toString(res));
+    }
+}