@dunlix thank you for your reply.
Great job people!
Does anybody know how to add advertisement widget/sidebar to the self-hosted version of the software?
If you want to do this with adsense, there is a plugin for this! https://github.com/dovydaskukalis/nodebb-plugin-google-adsense . I'm not sure if this provides it as a widget, though.
Maybe there are also other plugins for other ad providers.