Relocate and restore Figma content referenced in User Experience Guidelines for Mobile Extensions

[rebloor]

User Experience Guidelines for Mobile Extensions contained 2 references to figma content that was behind a login, specifically:

The extra mile > Themes > Second bullet
Match Firefox for Android’s [color usage](https://www.figma.com/proto/TebIgESfWmQkMcEmwNaYZl/Android?node-id=0%3A498&viewport=-4823%2C213%2C0.5&scaling=min-zoom&hide-ui=1) for light and dark themes.

The extra mile > Typography > Type scale
Follow the [Firefox for Android typographic scale](https://www.figma.com/proto/TebIgESfWmQkMcEmwNaYZl/Android?node-id=0%3A806&viewport=115%2C161%2C0.5&scaling=min-zoom).

The last mile > Components (Final paragraph)
'Components should match Firefox for Android color usage for light and dark themes. `