Add stacking context lesson in CSS positioning lesson block

We had a suggestion for a new issue here

https://github.com/freeCodeCamp/freeCodeCamp/issues/58411

We can add a new lesson after the z-index one about stacking context. Here are the MDN docs for that
https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Positioned_layout/Stacking_context

The new lesson can be called `What is the stacking context and how does it work?`

Here are the suggestions from the original issue on what to discuss in this lesson

- stacking contexts, including
- when they are created
- how they interact with the `z-index` property
- on what basis a stacking context takes priority over another
- etc.
- the `clear` property and how it works

This new lesson should be added after the z-index one here

https://www.freecodecamp.org/learn/responsive-web-design-v9/lecture-understanding-how-to-work-with-floats-and-positioning-in-css/what-is-the-z-index-property