docs(tutorial/step_14): replace broken web platform docs link

Replace broken [webplatform-animations][1] link with [mdn-animations][2].

The original link returns a 404. The closest match that works is
https://webplatform.github.io/docs/css/properties/animation. However,
the notice at the top of the page reads:

> The WebPlatform project, supported by various stewards between 2012
> and 2015, has been discontinued.

The CSS animations guide on MDN web docs is not only current, but also
more comprehensive.

[1]: https://docs.webplatform.org/wiki/css/properties/animations
[2]: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Animations/Using_CSS_animations

Closes #16294