docs-bug(Table): Add example with `rowspan`

### Documentation Feedback

There are many useful `Table` examples in the documentation. However, a common use case that’s missing is how to use `rowspan` to merge identical cells vertically.

For example, say we have the following data:

```ts
const ISOTOPE_DATA: PeriodicElement[] = [
{position: 1, element: 'Hydrogen', isotope: 'Protium', weight: 1.0078},
{position: 2, element: 'Hydrogen', isotope: 'Deuterium', weight: 2.0141},
{position: 3, element: 'Hydrogen', isotope: 'Tritium', weight: 3.016},
{position: 4, element: 'Helium', isotope: 'Helium-3', weight: 3.016},
{position: 5, element: 'Helium', isotope: 'Helium-4', weight: 4.0026},
];
```

We should be able to use it to create a table like this:

Element
Isotope
Weight


Hydrogen
Protium
1.0078


Deuterium
2.0141


Tritium
3.016


Helium
Helium-3
3.016


Helium-4
4.0026

This is related to https://github.com/angular/components/issues/11551, which is similar but for the `CdkTable` documentation.

### Affected documentation page

https://material.angular.dev/components/table/examples

从受影响的文档页面 https://material.angular.dev/components/table/examples 开始，查看现有的 Table 示例。添加一个使用所提供的 ISOTOPE_DATA 的示例，以演示使用 rowspan 垂直合并单元格，并匹配所展示的 Hydrogen 和 Helium 布局。完成的标准是页面能够清晰地解释并渲染这一用例。

由索引模型根据 Issue 内容生成。