Make header links non-selectable (#602)

Author: choldgraf

src/pydata_sphinx_theme/assets/styles/_base.scss

@@ -41,6 +41,7 @@ a {
     margin-left: 0.2em;
     text-decoration: none;
     transition: all 0.3s ease-out;
+    user-select: none;
 
     &:hover {
       opacity: 1;