Only "linked" parameters should have pointer cursor (#742)

If a parameter has no target to scroll to, it should not suggest
otherwise by changing the cursor to a pointer.  Since the scrolling is
done via JS, we should also change the cursor via JS.[1]

While we're at it, we also fix the potential JS error where we call the
`.offset()` method on an empty `jQuery` object.

[1] <https://github.com/php/doc-en/issues/2071#issuecomment-1354497367>

styles/theme-base.css

@@ -1220,9 +1220,6 @@ header.title {
 
 /* {{{ General styles (p, parameters, initializers, ...) */
 
-.refsect1 .parameter {
-    cursor:pointer;
-}
 .refsect1 dt {
     height:1.5rem;
 }