Feature: Add demo feature/markdown file for useIntersectionObserver

[francoisdillinger]

Pull Request Description

Description Added UseIntersectionObserverDemo.jsx and useIntersectionObserver.mdx files for documentation details.

Acceptance Criteria

• [x] The documentation for a hook/utility should show, as best as possible, how it works in action, so a complete usage example
• [x] The newly added item should be referenced in all other instances where other items are referenced in the docs/readme.

Demo:

[DavidHDev]

awesome stuff, thanks!