Introduction to Flexbox: Added slaying the dragon tutorial on flexbox (…

…#28600)

* Add slaying the dragon flexbox tutorial for intro to flexbox.

* Remove blank lines in between additional resources.

foundations/html_css/flexbox/flexbox-intro.md

@@ -86,5 +86,5 @@ The following questions are an opportunity to reflect on key topics in this less
 This section contains helpful links to related content. It isn't required, so consider it supplemental.
 
 - Interneting Is Hard has a tutorial on [modern CSS layouts with flexbox](https://internetingishard.netlify.app/html-and-css/flexbox/index.html).
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+- Slaying the dragon tutorial on [Flexbox in 8 minutes](https://youtu.be/phWxA89Dy94?si=UOXlsTa0BMfQYG3q).
 - For a more interactive explanation and example, try this [Scrim on Flexbox](https://scrimba.com/learn/flexbox/your-first-flexbox-layout-flexbox-tutorial-canLGCw). Note that this Scrim requires logging into Scrimba in order to view.