Fix search modal z-index to be on top of all docs showcased components

Search modal currently has a z-index set to 1030. That makes it under some showcased components in the documentation; especially the ones that stay displayed even if one clicks outside of it.

Here are two examples, popover and toast:

In _variables.scss we have this list of z-indexes:

$zindex-dropdown:                   1000 !default;
$zindex-sticky:                     1020 !default;
$zindex-fixed:                      1030 !default;
$zindex-offcanvas-backdrop:         1040 !default;
$zindex-offcanvas:                  1045 !default;
$zindex-modal-backdrop:             1050 !default;
$zindex-modal:                      1055 !default;
$zindex-popover:                    1070 !default;
$zindex-tooltip:                    1080 !default;
$zindex-toast:                      1090 !default;

I chose 2000 as a value here (maybe too extreme) just to be sure that it will always work in the future.

Live previews

• Popover - Live demo
• Toast - Live example

Thanks @louismaximepiton for this catch