add array flatten (#1951)

## Versions 

- [X] dev
- [ ] 3.0
- [ ] 2.1
- [ ] 2.0

## Languages

- [X] Chinese
- [X] English

## Docs Checklist

- [ ] Checked by AI
- [ ] Test Cases Built

docs/sql-manual/sql-functions/scalar-functions/array-functions/array-flatten.md

@@ -0,0 +1,63 @@
+---
+{
+    "title": "ARRAY_FLATTEN",
+    "language": "en"
+}
+---
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+## array_flatten
+
+array_flatten
+
+### description
+
+#### Syntax
+
+```sql
+ARRAY<T> array_flatten(ARRAY<ARRAY<T>> array1)
+```
+
+Flatten a multidimensional array into one dimension.
+
+### example
+
+```sql
+mysql> select array_flatten([[1,2,3],[4,5]]);
++--------------------------------+
+| array_flatten([[1,2,3],[4,5]]) |
++--------------------------------+
+| [1, 2, 3, 4, 5]                |
++--------------------------------+
+1 row in set (0.01 sec)
+
+mysql> select array_flatten([[[[[[1,2,3,4,5],[6,7],[8,9],[10,11],[12]],[[13]]],[[[14]]]]]]);
++-------------------------------------------------------------------------------+
+| array_flatten([[[[[[1,2,3,4,5],[6,7],[8,9],[10,11],[12]],[[13]]],[[[14]]]]]]) |
++-------------------------------------------------------------------------------+
+| [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]                               |
++-------------------------------------------------------------------------------+
+1 row in set (0.02 sec)
+```
+
+### keywords
+
+ARRAY,ARRAY_FLATTEN

i18n/zh-CN/docusaurus-plugin-content-docs/current/sql-manual/sql-functions/scalar-functions/array-functions/array-flatten.md

@@ -0,0 +1,63 @@
+---
+{
+    "title": "ARRAY_FLATTEN",
+    "language": "zh-CN"
+}
+---
+
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+## array_flatten
+
+array_flatten
+
+## 描述
+
+## 语法
+
+```sql
+ARRAY<T> array_flatten(ARRAY<ARRAY<T>> array1)
+```
+
+将多维数组展平成一维。
+
+## 举例
+
+```sql
+mysql> select array_flatten([[1,2,3],[4,5]]);
++--------------------------------+
+| array_flatten([[1,2,3],[4,5]]) |
++--------------------------------+
+| [1, 2, 3, 4, 5]                |
++--------------------------------+
+1 row in set (0.01 sec)
+
+mysql> select array_flatten([[[[[[1,2,3,4,5],[6,7],[8,9],[10,11],[12]],[[13]]],[[[14]]]]]]);
++-------------------------------------------------------------------------------+
+| array_flatten([[[[[[1,2,3,4,5],[6,7],[8,9],[10,11],[12]],[[13]]],[[[14]]]]]]) |
++-------------------------------------------------------------------------------+
+| [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]                               |
++-------------------------------------------------------------------------------+
+1 row in set (0.02 sec)
+```
+
+### keywords
+
+ARRAY,ARRAY_FLATTEN

sidebars.json

@@ -1221,6 +1221,7 @@
                                         "sql-manual/sql-functions/scalar-functions/array-functions/array-first-index",
                                         "sql-manual/sql-functions/scalar-functions/array-functions/array-last-index",
                                         "sql-manual/sql-functions/scalar-functions/array-functions/array-first",
+                                        "sql-manual/sql-functions/scalar-functions/array-functions/array-flatten",
                                         "sql-manual/sql-functions/scalar-functions/array-functions/array-last",
                                         "sql-manual/sql-functions/scalar-functions/array-functions/arrays-overlap",
                                         "sql-manual/sql-functions/scalar-functions/array-functions/array-count",