Yes, that would be the second listed feature then! If you find the Navigation button under the "Companion Forums Site" section in the Tightknit settings, you'll find the ability to add new navigation menu items.
If you want to embed your own web page as a sidebar item, the format will be:
<Your Sidebar Item Name> | <url> | <url-slug>
For example:
Terms of Service | https://www.my-site.com/tos | terms-of-service