ignore tt3 screenshot (#6790)

Doctom and Dotorg screenshots for this theme are different, so we need
to avoid syncing when updating the theme, unless we explicitly intend to
change the Dotcom image.

.sandbox-ignore

@@ -24,4 +24,5 @@ variations/**
 inc/headstart
 languages
 *.map
-.DS_Store
+.DS_Store
+**/*/twentytwentythree/screenshot.png