Display count of how many rows are selected on a data table

### Summary
Display an indication of how many rows are selected on a data table when using `rowSelection`.

Design task to consider this functionality.

### 💬 Description
When using the `rowSelection` functionality ([website example](https://design.sis.gov.uk/components/data-table/code#select-rows)), I think it would be useful to show a count of how many rows are selected. E.g. "3 out of 10 rows selected".

This would be particularly useful when using pagination, because currently, the "select all" checkbox only selects all rows on the current page. Some users may not realise this, but if a count of selected rows was visible, it would be clearer, because it would say e.g. "10 out of 40 rows selected".

Also, a user may want to select a certain number rows. They would have to count them one by one rather than just reading a number.

### 💰 Use value
To help avoid confusion with "select all" and allow users to quickly see how many rows they have selected.

Start with the linked data-table rowSelection example and review how selection behaves across pagination. No implementation file or test is named; done would require an agreed design and a visible selected-row count that makes the current selection scope clear.

Written by the indexing model from the issue text.