Add link underline color to 1.4.11 (#3913)

Adds an example where link and non-link text are both white on a black
page background. The only way to identify the link is via a dull-red
underline that has a contrast of less than 3:1.

[Rendered
page](https://raw.githack.com/w3c/wcag/add-underline-color-to-1.4.11/understanding/21/non-text-contrast.html)
(check the User Interface Component Examples table)

Closes #3987

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Co-authored-by: Mike Gower <mikegower@gmail.com>

Author: fstrr