[feat](bi) support default_init_catalog user property for external catalog (#2359)

Add docs for pr: apache/doris#49658

## Versions 

- [x] dev
- [ ] 3.0
- [ ] 2.1
- [ ] 2.0

## Languages

- [x] Chinese
- [x] English

## Docs Checklist

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

Author: cambyzju

docs/lakehouse/catalog-overview.md

@@ -154,6 +154,17 @@ jdbc:mysql://host:9030/iceberg_catalog.iceberg_db
 
 The fixed name for the built-in catalog is `internal`. The switching method is the same as for external catalogs.
 
+### Default Catalog
+The user attribute `default_init_catalog` is used to set the default catalog for a specific user. Once set, when the specified user connects to Doris, they will automatically switch to the set catalog.
+
+```sql
+SET PROPERTY default_init_catalog=hive_catalog;
+```
+
+Note 1: If the catalog has been explicitly specified in the MySQL command line or JDBC connection strings, then the specified catalog will be used, and the `default_init_catalog` user attribute will not take effect.
+Note 2: If the catalog set by the user attribute `default_init_catalog` no longer exists, it will automatically switch to the default `internal` catalog.
+Note 3: This feature takes effect starting from version v3.1.x.
+
 ### Simple Query
 
 You can query tables in external catalogs using any SQL statement supported by Doris.

i18n/zh-CN/docusaurus-plugin-content-docs/current/lakehouse/catalog-overview.md

@@ -153,6 +153,18 @@ jdbc:mysql://host:9030/iceberg_catalog.iceberg_db
 
 内置数据目录的固定名称为 `internal`。切换方式和外部数据目录一致。
 
+### 默认数据目录
+
+通过用户属性 `default_init_catalog`，为指定用户设置默认的数据目录。设置完成后，当指定用户连接 Doris 时，会自动切换到设置的数据目录。
+
+```sql
+SET PROPERTY default_init_catalog=hive_catalog;
+```
+
+注意1：如果 MySQL 命令行或 JDBC 连接串中已经明确指定了数据目录，则以指定的为准，`default_init_catalog` 用户属性不生效；
+注意2：如果用户属性 `default_init_catalog` 设置的数据目录已经不存在，则自动切换到默认的 `internal` 数据目录；
+注意3：该功能从 v3.1.x 版本开始生效；
+
 ### 简单查询
 
 可以通过 Doris 支持的任意 SQL 语句查询外部数据目录中的表。