This plugin shows categories on the page sidebar on all discovery list pages (latest, new, unread but not on the single topic page, and categories).
Blocks can be ordered in the plugin settings. Speaking of which, there are these settings under
Admin > Settings > Plugins:
sidebar enableenables the display of the sidebar
sidebar custom contentwhere you can enter arbitrary HTML content to show on the sidebar
To install plugin: Install Plugins in Discourse
The plugins works on desktop only, and there are no plans to add it to the mobile view.
Important note. Despite the clarity, this example adds duplication of categories that already exist elsewhere on this page. Of course, we can hide some navigation elements to avoid duplication, but this will entail even more changes.
I think this could be a good example of using a sidebar. You can clone the plugin and use it for your needs (changing the code for your specific tasks).