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/cpp/chapter_searching/two_sum.cpp

@@ -0,0 +1,54 @@
+/**
+ * File: two_sum.cpp
+ * Created Time: 2022-11-25
+ * Author: krahets (krahets@163.com)
+ */
+
+#include "../utils/common.hpp"
+
+/* 方法一：ブルートフォース列挙 */
+vector<int> twoSumBruteForce(vector<int> &nums, int target) {
+    int size = nums.size();
+    // 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 {i, j};
+        }
+    }
+    return {};
+}
+
+/* 方法二：補助ハッシュテーブル */
+vector<int> twoSumHashTable(vector<int> &nums, int target) {
+    int size = nums.size();
+    // 補助ハッシュテーブル、空間計算量はO(n)
+    unordered_map<int, int> dic;
+    // 単層ループ、時間計算量はO(n)
+    for (int i = 0; i < size; i++) {
+        if (dic.find(target - nums[i]) != dic.end()) {
+            return {dic[target - nums[i]], i};
+        }
+        dic.emplace(nums[i], i);
+    }
+    return {};
+}
+
+/* ドライバコード */
+int main() {
+    // ======= テストケース =======
+    vector<int> nums = {2, 7, 11, 15};
+    int target = 13;
+
+    // ====== ドライバコード ======
+    // 方法一
+    vector<int> res = twoSumBruteForce(nums, target);
+    cout << "方法一 res = ";
+    printVector(res);
+    // 方法二
+    res = twoSumHashTable(nums, target);
+    cout << "方法二 res = ";
+    printVector(res);
+
+    return 0;
+}