docs: Add docs for flatMap

docs/.vitepress/zh_hans.mts

@@ -69,6 +69,7 @@ function sidebar(): DefaultTheme.Sidebar {
             },
             { text: "fill", link: "/zh_hans/reference/array/fill" },
             { text: "toFilled", link: "/zh_hans/reference/array/toFilled" },
+            { text: "flatMap", link: "/reference/array/flatMap" },
             { text: "flatten", link: "/zh_hans/reference/array/flatten" },
             {
               text: "forEachRight",

docs/ko/reference/array/flatMap.md

@@ -8,7 +8,7 @@ JavaScript 언어에 포함된 [Array#flat](https://developer.mozilla.org/en-US/
 
 ```typescript
 function flatMap<T, U, D extends number = 1>(
-  arr: readonly T[],
+  arr: T[],
   iteratee: (item: T) => U,
   depth?: D
 ): Array<FlatArray<U[], D>>;

docs/reference/array/flatMap.md

@@ -8,7 +8,7 @@ It works the same as if you called [Array#flat](https://developer.mozilla.org/en
 
 ```typescript
 function flatMap<T, U, D extends number = 1>(
-  arr: readonly T[],
+  arr: T[],
   iteratee: (item: T) => U,
   depth?: D
 ): Array<FlatArray<U[], D>>;

docs/zh_hans/reference/array/flatMap.md

@@ -0,0 +1,40 @@
+# flatMap
+
+将嵌套数组的每个元素映射到给定的迭代函数，然后将其展平到所需的深度。
+
+其工作方式与在 JavaScript 语言中调用 [Array#flat](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat) 和 [Array#map](https://developer.mozilla.org/ko/docs/Web/JavaScript/Reference/Global_Objects/Array/map) 时相同，即 `map(iteratee).flat(depth)`，但速度更快。
+
+## 签名
+
+```typescript
+function flatMap<T, U, D extends number = 1>(
+  arr: T[],
+  iteratee: (item: T) => U,
+  depth?: D
+): Array<FlatArray<U[], D>>;
+```
+
+### 参数
+
+- `arr` (`T[]`): 要展平的数组。
+- `iteratee` (`T[]`): 映射每个数组元素的函数。
+- `depth` (`D`): 要展平的深度，默认为 1。
+
+### 返回值
+
+(`Array<FlatArray<U[], D>>`): 一个新数组，其中每个元素都已映射并展平到所需的深度。
+
+## 示例
+
+```typescript
+const array = [1, 2, 3];
+
+const result1 = flatMap(array, item => [item, item], 1);
+// 返回 [1, 1, 2, 2, 3, 3]
+
+const result2 = flatMap(array, item => [[item, item]], 2);
+// 返回 [1, 1, 2, 2, 3, 3]
+
+const result3 = flatMap(array, item => [[[item, item]]], 3);
+// 返回 [1, 1, 2, 2, 3, 3]
+```