Advanced HTML and CSS transitions: add additional article

[fadingmorsecode]

Because

This PR aims to introduce stacking context in a way that is less intimidating and easier to understand. The additional article is a less formal introduction to stacking context with analogies and examples that may be easily understood by readers. The PR still keeps the MDN article of stacking context but readers will hopefully have a better understanding while reading it.

This PR

• Adds additional article to stacking context in assignment

Pull Request Requirements

• I have thoroughly read and understand The Odin Project curriculum contributing guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
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[MaoShizhong]

Thanks for this @fadingmorsecode.

I like the Josh Comeau article for sure and think it would be a great add to the lesson. I do actually like it more than the currently linked MDN page, though we should still include the MDN page as it's more of a reference than an article explaining concepts. I feel having them both in the assignments would be a little much just for that point.

Could you move the MDN article out of the assignment list and into the additional resources as its own item?