Love the idea of related articles, but the complexities to calculate which ones relate will probably mean HubSpot couldn't commit to that scale of project.
Shamelessly, I'd recommend Bibblio. It can be added to your template as a code snippet and automatically suggests related articles using AI - so no editorial overhead. Category grouping could be possible too. It's free for small sites or testing.
The ability to add related articles is now live in the Knowledge base. To activate Related Articles, Go to Settings -> Service -> Knowledge Base and you will see a Related Articles setting. Simply enable it for related articles to appear.