migrate performance examples to ds repo

[designakt]

The examples are currently broken... probably because they are hosted on my GitHub page... so moving them over should solve that. (need to check if that works for the images I load from somewhere else...)
https://design.firefox.com/photon/introduction/design-for-performance.html

[aminalhazwani]

Initially the examples were hosted on Markus's github pages but these pages were served through https so they were not visible on our non-secure website. As a temporary solution I copied those examples to my github pages which were served with http because I have a custom domain as our design system website has. Now that the design system website is served with https we can regrab the examples from Markus's https github pages. After the refactor we can better figure how to host those examples ourselves.