doc: underline links

The current use of color only to differentiate links fails WCAG A
accessibility standards.

Possible solutions are:

1. Using a text indicator to show that the text is a link.
2. Using additional cues (such as underlining).
3. Providing color contrast of 3:1 or greater with surrounding text
   (not the background--the adjacent text!) along with visual cues
   on hover.

The solution here implements the second option.

Ref: https://www.w3.org/WAI/WCAG21/Understanding/use-of-color.html
Ref: https://www.w3.org/WAI/WCAG21/Techniques/failures/F73
Ref: https://www.w3.org/WAI/WCAG21/Techniques/general/G182
PR-URL: #50481
Refs: https://www.w3.org/WAI/WCAG21/Understanding/use-of-color.html
Refs: https://www.w3.org/WAI/WCAG21/Techniques/failures/F73
Refs: https://www.w3.org/WAI/WCAG21/Techniques/general/G182
Reviewed-By: Claudio Wunder <cwunder@gnome.org>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Tierney Cyren <hello@bnb.im>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api_assets/style.css

@@ -136,7 +136,6 @@ a:link,
 a:active,
 a:visited {
   color: var(--color-links);
-  text-decoration: none;
   border-radius: 2px;
   padding: 1px 3px;
 }