docs: css: add .strike class

Add a custom CSS class to support strike-through text without depending on
a specific theme.

(cherry picked from commit 7193ed0b)

docs/_static/css/custom.css

+.strike {
+	text-decoration: line-through;
+}
+
 /*
 	This fixes the vertical position of list markers when the first
 	element in the <li> is a <pre> block