The document describes:
Is it possible to display the navigation link in the content area?
The side nav item is part of the side nav component, which is intended to display a structural element for side navigation.
If you want to have a normal navigational link somewhere in your application, you can simply use an Anchor or RouterLink. The first one represents the anchor element and takes a string (href), while the second one is more intended for your use case (to link directly to a routed component)
See details here: How to navigate to a view in Flow | Vaadin and/or here How to implement routing and navigation in Vaadin
In general, you you could theoretically use web components, that are intended to be used as child elements of other web components as a standalone variant, but that might lead to issues with initializing or styling, so I would not recommend that, as long as the documentation does not state something different.