docs/sphinx: add 's' keyboard binding to focus search

This is pretty ubiquitous. ('/' is already taken by some browsers for
quick search)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

docs/sphinx-static/custom.js

@@ -0,0 +1,9 @@
+document.addEventListener('keydown', (event) => {
+    // find a better way to look it up?
+    let search_input = document.getElementsByName('q')[0];
+
+    if (event.code === 'KeyS' && document.activeElement !== search_input) {
+        event.preventDefault();
+        search_input.focus();
+    }
+});