Content using `book` layout sometimes not visible

[zmoon]

Between the mobile breakpoint and ~ 900 px, the content is not all visible on the screen without scrolling horizontally.

900: https://screenfly.org/#u=https%3A//fortran-lang.org/learn/quickstart/arrays_strings&w=900&h=900
600: https://screenfly.org/#u=https%3A//fortran-lang.org/learn/quickstart/arrays_strings&w=600&h=900

[certik]

We should move the left navigation panel to the top, then the horizontal area can be used more effectively.

[awvwgk]

This should be resolved with the migration to the new webpage.