Issue on docs bitmap-union-count

- Page Link: https://doris.apache.org/zh-CN/docs/sql-manual/sql-functions/aggregate-functions/bitmap-union-count

- Version: 2.1

- Reason: 可以加一个最佳实践，如下
```sql
-- 在窗口函数中，bitmap_union_count相比bitmap_count(bitmap_union(x))性能更优，推荐使用bitmap_union_count

-- bitmap_count(bitmap_union(x)) 结合窗口函数，计算累计指标
BITMAP_COUNT(
BITMAP_UNION(1min_bitmap) OVER (
ORDER BY
minute_time
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
)
)

-- bitmap_union_count 结合窗口函数，计算累计指标
BITMAP_UNION_COUNT(1min_bitmap) OVER (
ORDER BY
minute_time
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
)
```

- May I contribute to this pull request? Our company has been frequently using Doris lately. If given the chance, I would also like to gradually become involved in the community.（如果确认该Issue的话，可以由我来提交PR吗，我很希望参与到doris社区当中）

- ScreenShot:

Open the linked Chinese bitmap-union-count documentation page and review its existing examples and wording. Add the requested best-practice comparison and cumulative-metric window-function examples, then verify the rendered page and SQL formatting; done means the guidance is clear and both examples appear correctly.

Written by the indexing model from the issue text.