Display the nav_links in the mobile menu

Hi! :wave:

First of all, thank you for open sourcing this docs theme. We've used it in Front-Commerce's documentation and it is really great!

However, I've noticed that there's an issue when displaying the `nav_links` on mobile. They should appear above the sidebar but are hidden behind the header. It is displayed correctly though when you start in desktop mode and resize to mobile.

This can be seen in Apollo Docs : https://www.apollographql.com/docs/

We've made a fix on our side, but I'm not sure that the PR would be OK for you since our fix uses flexbox and I've noticed that it does not seem to appear in your codebase. If it's OK for you though, I could make a PR later on.

Our fix: https://github.com/front-commerce/developers.front-commerce.com/pull/18/commits/c1228033a3e5766ef8486d048b4d7dd9a3ed1429
A working preview: https://deploy-preview-18--elastic-austin-cf9d40.netlify.com/docs/

Reproduce the issue by loading the mobile menu directly, then compare it with resizing from desktop and inspect the theme styles controlling nav_links, the header, and the sidebar. Done means nav_links appear above the sidebar on an initial mobile load without regressing the desktop layout.

Written by the indexing model from the issue text.