Fix unreadable links in important alerts and the sign-in cover page (#2919)

docs/content/ui/components/alert.mdx

@@ -284,6 +284,12 @@ You can also use other elements, like icons and dismissible buttons, with this t
 </div>
 </Example>
 
+Links work here too. Both `alert-link` and `alert-action` follow the alert color, so they turn white on the solid background.
+
+<Example>
+<Alert variant="important" color="danger" title="An error occurred!" action="Try again" /> <Alert variant="important" color="info" title="Your plan renews next week." link="See the details" />
+</Example>
+
 ## Custom alert color
 
 You're not limited to the 4 default alert colors. You can use any [base color](/ui/base/colors) you want.