Left-nav formatting issue when `sidebar_search_disable`

For the user guide, set `sidebar_search_disable: true` and visit the docs, then you'll see the following on a wide enough display:

> <img width="300" alt="Screen Shot 2023-08-10 at 17 52 45" src="https://github.com/google/docsy/assets/4140793/58c9599c-bb70-4f41-bb78-5167adb83508">

Space is missing at the top.

Related:

- #1657