Remove shortcut indicator search dialog on small screens (#1947)

Closes #1944.

Alternative to #1945. While #1945 removes the shortcut completely, I've
[suggested that it actually has
value](#1945 (comment))
and should only be removed on smaller screens. That's what this PR
implements. It shows the shortcuts if the window size is at least "md"
i.e. >=768px (I figured, this is a reasonable threshold - see discussion
below, but if you think other values are better, that's perfectly fine).

See pull requests for screenshots.

src/pydata_sphinx_theme/assets/styles/components/_search.scss

@@ -61,9 +61,13 @@
 
   // Shows off the keyboard shortcuts for the button
   .search-button__kbd-shortcut {
-    display: flex;
+    display: none;
     margin-inline-end: 0.5rem;
     color: var(--pst-color-border);
+
+    @include media-breakpoint-up(md) {
+      display: flex;
+    }
   }
 }