CSS examples: example for z-index

[veekas]

This updates the interactive demo for the z-index page. It is meant to somewhat mirror the current example on the z-index page.

Current example on MDN

Proposed, interactive example

Unfortunately, my linter added 4-space indents to the html that wasn't indented already. Sorry if that makes it more difficult to review the code.

Closes #554

[welcome]

💖 Thanks for opening this pull request! 💖
Here is a list of things that will help get it across the finish line: - If this is a new or updated CSS interactive example, please ensure that you followed the CSS styleguide - If this is a new or updated JavaScript interactive example, please ensure that you followed the JavaScript styleguide - If your changes affects any of the steps in our contribution docs, please also make the relevant changes there.

[wbamberg]

This looks good! Thanks for your contribution.

There's a duplicated example choice, and I don't think this example belongs in "transitions" - perhaps "positioned-layout" fits better?

[wbamberg]

This choice is a duplicate of the one before (both z-index: 3;). I think you could just remove one of them.

[veekas]

Whoops! Fixed.

[wbamberg]

Looks good, thanks for your contribution @veekas !

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉