Instant Search: improve navigation in Search customization screen

> Once you open the customize search page (not in the customizer) there’s no clear way to exit it. The sidebar is collapsed and anyone not familiar with the icons would likely be lost navigating away.

> if you click “Customize your Search experience”, everything seems to work fine on the customization page, except there’s no way to back out of that page unless you use your browser’s Back button or click on a WP Admin menu item.

*****

https://user-images.githubusercontent.com/426388/151796164-c2907ef2-2a28-476b-991b-6f564d623d17.mov

*****
Reported here:
- p8oabR-Nm-p2#comment-5880
- p8oabR-Nm-p2#comment-5961

Reproduce the issue by opening the Instant Search customization screen through “Customize your Search experience,” then inspect how the screen is entered and how WP Admin navigation is presented. Done means users have an obvious way to leave the customization screen without relying on the browser Back button or a WP Admin menu item.

Written by the indexing model from the issue text.