Divider: Improve default behavior when in vertical orientation

[mirka]

Follow-up to #38842

What?

• Conditionally renders the divider as an inline element when the orientation is vertical.
• Improves stories and documentation.

Why?

This ensures that the divider is visible "as expected" in both orientations when inside a flow layout container (not flex). Initially I was only going to improve the docs to demonstrate proper usage in the flex layout case, but I realized that we can also add a conditional display declaration so it just works out of the box.

I'm inclined to classify this as a non-breaking change, because any proper usage of the vertical divider should already be display: inline or inside a flex container (in which case the inline won't matter).

Testing Instructions

npm run storybook:dev and check out the stories for Divider.

• The vertical orientation works without a flex container.
• Inside a flex container, it still works as expected as long as it's stretch aligned.

[ciampo]

🎉 🚀

Much better!