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_computational_complexity/iteration.cpp

@@ -0,0 +1,76 @@
+/**
+ * File: iteration.cpp
+ * Created Time: 2023-08-24
+ * Author: krahets (krahets@163.com)
+ */
+
+#include "../utils/common.hpp"
+
+/* for ループ */
+int forLoop(int n) {
+    int res = 0;
+    // 1, 2, ..., n-1, n の合計をループ計算
+    for (int i = 1; i <= n; ++i) {
+        res += i;
+    }
+    return res;
+}
+
+/* while ループ */
+int whileLoop(int n) {
+    int res = 0;
+    int i = 1; // 条件変数を初期化
+    // 1, 2, ..., n-1, n の合計をループ計算
+    while (i <= n) {
+        res += i;
+        i++; // 条件変数を更新
+    }
+    return res;
+}
+
+/* while ループ（2つの更新） */
+int whileLoopII(int n) {
+    int res = 0;
+    int i = 1; // 条件変数を初期化
+    // 1, 4, 10, ... の合計をループ計算
+    while (i <= n) {
+        res += i;
+        // 条件変数を更新
+        i++;
+        i *= 2;
+    }
+    return res;
+}
+
+/* 2重 for ループ */
+string nestedForLoop(int n) {
+    ostringstream res;
+    // ループ i = 1, 2, ..., n-1, n
+    for (int i = 1; i <= n; ++i) {
+        // ループ j = 1, 2, ..., n-1, n
+        for (int j = 1; j <= n; ++j) {
+            res << "(" << i << ", " << j << "), ";
+        }
+    }
+    return res.str();
+}
+
+/* ドライバーコード */
+int main() {
+    int n = 5;
+    int res;
+
+    res = forLoop(n);
+    cout << "\nfor ループの合計結果 res = " << res << endl;
+
+    res = whileLoop(n);
+    cout << "\nwhile ループの合計結果 res = " << res << endl;
+
+    res = whileLoopII(n);
+    cout << "\nwhile ループ（2つの更新）の合計結果 res = " << res << endl;
+
+    string resStr = nestedForLoop(n);
+    cout << "\n2重 for ループ走査の結果 = " << resStr << endl;
+
+    return 0;
+}