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.

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

ja/codes/java/chapter_backtracking/permutations_i.java

@@ -0,0 +1,51 @@
+/**
+ * File: permutations_i.java
+ * Created Time: 2023-04-24
+ * Author: krahets (krahets@163.com)
+ */
+
+package chapter_backtracking;
+
+import java.util.*;
+
+public class permutations_i {
+    /* バックトラッキングアルゴリズム：順列 I */
+    public static void backtrack(List<Integer> state, int[] choices, boolean[] selected, List<List<Integer>> res) {
+        // 状態の長さが要素数と等しくなったら、解を記録
+        if (state.size() == choices.length) {
+            res.add(new ArrayList<Integer>(state));
+            return;
+        }
+        // すべての選択肢を走査
+        for (int i = 0; i < choices.length; i++) {
+            int choice = choices[i];
+            // 剪定：要素の重複選択を許可しない
+            if (!selected[i]) {
+                // 試行：選択を行い、状態を更新
+                selected[i] = true;
+                state.add(choice);
+                // 次のラウンドの選択に進む
+                backtrack(state, choices, selected, res);
+                // 回退：選択を取り消し、前の状態に復元
+                selected[i] = false;
+                state.remove(state.size() - 1);
+            }
+        }
+    }
+
+    /* 順列 I */
+    static List<List<Integer>> permutationsI(int[] nums) {
+        List<List<Integer>> res = new ArrayList<List<Integer>>();
+        backtrack(new ArrayList<Integer>(), nums, new boolean[nums.length], res);
+        return res;
+    }
+
+    public static void main(String[] args) {
+        int[] nums = { 1, 2, 3 };
+
+        List<List<Integer>> res = permutationsI(nums);
+
+        System.out.println("入力配列 nums = " + Arrays.toString(nums));
+        System.out.println("すべての順列 res = " + res);
+    }
+}