Add stacking context lesson in CSS positioning lesson block

[jdwilkin4]

We had a suggestion for a new issue here

freeCodeCamp/freeCodeCamp#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