[EmptyState] Add documentation for onAction prop usage

[TheGreatSimo]

WHY are these changes introduced?

Updates the documentation to include the onAction prop for the EmptyState component.

While using the EmptyState component, I couldn't find how to handle clicks on the primary button. After checking the source code, I found the onAction prop. I’ve updated the documentation to include onAction so others can find it easily..

Fixes #810 #810

WHAT is this pull request doing?

Adds documentation to clarify how to use the onAction prop in the EmptyState component.
Provides a sample code implementation showing how to handle button actions using onAction.

Before: The onAction prop was available in the EmptyState component, but there was no clear mention in the documentation.

After: The documentation now includes examples showing how to use onAction for custom button behavior.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

🎩 checklist

• Tested a snapshot
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide