[UX] When there is only one menu item in a section, don't list

[jenlampton]

we have some sections of the admin menu that lead to landing pages that list everything in that section. On pages like /admin and /admin/config this is particularly useful. But if there were only a single link on that landing page, it could be frustrating to click there, then need to click a level deeper immediately.

Search, for example, has only a single sub-page in core.

Content has only a single sub-page in core.

Administration also has only a single sub-page in core.

I'm not sure how hard it would be to do, but I would love it if when there is only a single sub-item it went directly to that section.