Allow `p-table--mobile-card` headings to wrap

## Pattern to amend

`p-table--mobile-card `

## Visual


```
.p-table--mobile-card tbody th {
white-space: normal;
}
```

## Context

Often the heading is not fully readable. This defeats the purpose of breaking the table out into cards.

See conversation on this pr: https://github.com/canonical/ubuntu.com/pull/13049

Search the `p-table--mobile-card` styles and inspect the `tbody th` rule shown in the issue. Compare the mobile-card rendering with the linked visual and conversation; done means table headings wrap and remain fully readable on mobile.

Written by the indexing model from the issue text.