This is a regular content page, with content placed in the left sidebar.
The left sidebar content is defined in the content file, and in this case is provided by the {{< sidebar/subnav >}} shortcode.
Usage
The content file looks something like this:
---
title: Content page with left sidebar
layout: content-page
…
---
{{< main >}}
Main content goes here…
{{< /main >}}
{{< sidebar left >}}
{{< sidebar/subnav >}}
{{< /sidebar >}}