There’s a slight illusion here — they didn’t create one nav bar to use universally, they’ve created the nav bar on their site (aurelia.io) and then duplicated it within their Discourse site (https://discourse.aurelia.io/) so it looks identical.
So the steps would be:
- Create your main site
- Setup Discourse
- Create a theme in Discourse to match your main site’s nav
Some resources for building Discourse themes:
Beginner's guide to using Discourse themes
Developer’s guide to Discourse Themes