fix(a11y): underline content links to avoid color-only differentiation

RTD theme removes underlines from links (text-decoration: none), so
content links are distinguished from surrounding text by color alone.
The contrast between link blue and body text is below 3:1, which
fails WCAG 1.4.1 / BITV 9.1.4.1 when color is the sole indicator.

Adding text-decoration: underline provides a non-color visual cue,
satisfying the requirement without changing any color values.

Fixes #9673

Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

Author: skjnldsv

_shared_assets/static/custom.css

@@ -17,6 +17,14 @@ h6 {
 	background-color: #0082c9;
 }
 
+/* Links in the content area must be distinguishable from body text
+   without relying on color alone (WCAG 1.4.1 / BITV 9.1.4.1).
+   RTD theme sets text-decoration: none, so underline is the simplest
+   non-color indicator that satisfies the requirement. */
+.rst-content a {
+	text-decoration: underline;
+}
+
 /* Reduce size of logo in top left */
 .wy-side-nav-search > a img.logo {
 	max-width: 180px;