docs: add array_agg_if aggregate function doc

docs/sql-manual/sql-functions/aggregate-functions/array-agg-if.md

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+---
+{
+    "title": "ARRAY_AGG_IF",
+    "language": "en",
+    "description": "Concatenates the values of rows that meet a condition into an array, which can be used for conditional pivoting of rows into columns."
+}
+---
+
+## Description
+
+Concatenates the values (including null values) in a column into an array for the rows whose condition is true. Rows whose condition is false or null are skipped entirely.
+
+## Syntax
+
+```sql
+ARRAY_AGG_IF(<cond>, <col>)
+```
+
+## Parameters
+
+| Parameter | Description |
+| -- | -- |
+| `<cond>` | A BOOLEAN expression that determines whether a row is collected. Rows where the condition is false or null are skipped. |
+| `<col>` | An expression that determines the values to be placed into the array. Supported types: Bool, TinyInt, SmallInt, Integer, BigInt, LargeInt, Float, Double, Decimal, Date, Datetime, TimestampNs, Timestamptz, IPV4, IPV6, String, Array, Map, Struct. |
+
+## Return Value
+
+Returns a value of ARRAY type. Special cases:
+
+- The order of elements in the array is not guaranteed.
+- Null elements are kept: when the condition is true, a null value in `<col>` still becomes a null element in the result.
+- Returns an empty array when no row satisfies the condition.
+
+## Example
+
+```sql
+-- setup
+CREATE TABLE test_doris_array_agg_if (
+	c1 INT,
+	c2 INT
+) DISTRIBUTED BY HASH(c1) BUCKETS 1
+PROPERTIES ("replication_num" = "1");
+INSERT INTO test_doris_array_agg_if VALUES (1, 10), (1, 20), (1, 30), (2, 100), (2, 200), (3, NULL);
+```
+
+```sql
+-- collect the values greater than 15 per group; the NULL-valued row of c1=3 is skipped
+select c1, array_agg_if(c2 > 15, c2) from test_doris_array_agg_if group by c1;
+```
+
+```text
++------+------------------------+
+| c1   | array_agg_if(c2 > 15, c2) |
++------+------------------------+
+|    1 | [20, 30]               |
+|    2 | [100, 200]             |
+|    3 | []                     |
++------+------------------------+
+```
+
+```sql
+-- a null element is kept when its row satisfies the condition
+select c1, array_agg_if(c1 > 0, c2) from test_doris_array_agg_if group by c1;
+```
+
+```text
++------+----------------------+
+| c1   | array_agg_if(c1 > 0, c2) |
++------+----------------------+
+|    1 | [10, 20, 30]         |
+|    2 | [100, 200]           |
+|    3 | [null]               |
++------+----------------------+
+```

i18n/zh-CN/docusaurus-plugin-content-docs/current/sql-manual/sql-functions/aggregate-functions/array-agg-if.md

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+---
+{
+    "title": "ARRAY_AGG_IF",
+    "language": "zh-CN",
+    "description": "将满足条件的行中的值串联成一个数组，可以用于多行转一行（行转列）的条件过滤。"
+}
+---
+
+## 描述
+
+对于满足条件的行，将其一列中的值（包括空值 null）串联成一个数组。条件为 false 或 null 的行会被整体跳过（不贡献元素，也不贡献 null）。
+
+## 语法
+
+```sql
+ARRAY_AGG_IF(<cond>, <col>)
+```
+
+## 参数
+
+| 参数 | 说明 |
+| -- | -- |
+| `<cond>` | BOOLEAN 类型表达式，决定该行是否被收集。条件为 false 或 null 时该行被跳过。 |
+| `<col>` | 确定要放入数组的值的表达式，支持类型为 Bool，TinyInt，SmallInt，Integer，BigInt，LargeInt，Float，Double，Decimal，Date，Datetime，TimestampNs，Timestamptz，IPV4，IPV6，String，Array，Map，Struct。|
+
+## 返回值
+
+返回 ARRAY 类型的值，特殊情况：
+
+- 数组中元素不保证顺序。
+- 保留 null 元素：当条件为 true 时，`<col>` 的 null 值仍会成为结果数组中的一个 null 元素。
+- 没有任何行满足条件时，返回空数组。
+
+## 举例
+
+```sql
+-- setup
+CREATE TABLE test_doris_array_agg_if (
+	c1 INT,
+	c2 INT
+) DISTRIBUTED BY HASH(c1) BUCKETS 1
+PROPERTIES ("replication_num" = "1");
+INSERT INTO test_doris_array_agg_if VALUES (1, 10), (1, 20), (1, 30), (2, 100), (2, 200), (3, NULL);
+```
+
+```sql
+-- 每组收集大于 15 的值；c1=3 的行条件为 null，被跳过
+select c1, array_agg_if(c2 > 15, c2) from test_doris_array_agg_if group by c1;
+```
+
+```text
++------+------------------------+
+| c1   | array_agg_if(c2 > 15, c2) |
++------+------------------------+
+|    1 | [20, 30]               |
+|    2 | [100, 200]             |
+|    3 | []                     |
++------+------------------------+
+```
+
+```sql
+-- 行满足条件时，null 元素被保留
+select c1, array_agg_if(c1 > 0, c2) from test_doris_array_agg_if group by c1;
+```
+
+```text
++------+----------------------+
+| c1   | array_agg_if(c1 > 0, c2) |
++------+----------------------+
+|    1 | [10, 20, 30]         |
+|    2 | [100, 200]           |
+|    3 | [null]               |
++------+----------------------+
+```