Card: Add story and description to highlight CardMedia usage

[mirka]

What problem does this address?

An important feature of the CardMedia component is that it is overflow: hidden, meaning that it will make its content nicely match the border-radius of its parent Card. This can be hard to discover for a couple reasons:

• The user is not necessarily looking to wrap "media" like images or video. Sometimes it is just content with a background color (related: the "smooth the corners" issue in Reviewing Global Styles main panel #38934).
• The Storybook story for Card includes a usage of CardMedia, but it does not show rounded corners because it's placed in the middle of the card.

What is your proposed solution?

• The non-media use case should at least be mentioned in the lede of the CardMedia readme.
• There should be a story that makes it obvious that by using CardMedia, corners will be rounded to match the Card.

[github-actions]

Hi,
This issue has gone 180 days without any activity. This means it is time for a check-in to make sure it is still relevant. If you are still experiencing this issue with the latest versions, you can help the project by responding to confirm the problem and by providing any updated reproduction steps.
Thanks for helping out.

[kathrynwp]

Heya @mirka, I'm just popping round to check whether this is still an active/planned feature, and if so, whether there's anything that can be done to move it along. Thanks for having a look! (I've left the automatically added "Needs Testing" label for now.)

[mburridge]

What needs doing here? What should be added to the readme? Happy to action this with some guidance from the component authors or other interested parties.

[mirka]

@mburridge Thanks for the nudge, I went ahead and added the changes in #50074.