fix(docs/a11y): fix lack of contrast in on hovered links in footer

This commit changes the hover color of links in the footer to keep it
accessible.

The color will be the exact same as the un-hovered version. This is in
line with the other links in the documentation. The links _do_ get
underlined when hovered, so there is still a visual distinction
between the states.

I've added the variable in the `footer` section instead of in `:root`
because the variable I assign it to (`--ifm-footer-link-color`) gets
overridden closer to the footer, so if you set it in `:root`, you end
up with the wrong value.

addresses and closes https://github.com/Unleash/unleash.github.io/issues/8

website/src/css/custom.css

@@ -20,6 +20,10 @@
     --navbar-link-color: #122d33;
 }
 
+footer {
+    --ifm-footer-link-hover-color: var(--ifm-footer-link-color);
+}
+
 [class^='docTitle'] {
     font-size: 2.5rem !important;
 }