[Feature request] Add support for displaying an icon in the Section Intro

[seangolob]

Hi PB Team!

We have a few pages that are coming down the pipe that have a Section Intro with a icon.

Do you think we could create a leadingVisual prop to handle this use case?

As always, we can implement this in the consuming application but it would be nice to have spacing defined at the PB level so we can get consistency across implementations.

CC @natalie-iv

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