Disable smooth scrolling

It was not enabled before the docsy update and it messes with
the TOC highlighting during the transition.

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

Author: zecakeh

assets/scss/_variables_project.scss

@@ -49,3 +49,6 @@ $td-enable-google-fonts: false;
  * The font itself is loaded via stylesheet link layouts/partials/hooks/head-end.html.
  */
 $font-family-sans-serif: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
+
+// Disable smooth scrolling as it makes TOC highlighting jump during the transition.
+$enable-smooth-scroll: false;