Links on the top bar to helix pages open a new tab

This shouldn't be due to proper use of [this](https://github.com/adobe/aio-theme#external-links) feature. It is probably a bug in the framework we're using.

Start by reproducing the top-bar links to Helix pages and reviewing the linked aio-theme external-links guidance. No repository files or tests are named, so locate the navigation configuration and framework behavior before deciding where the issue lives. Done means Helix links open in the current tab without breaking intended external-link behavior.

Written by the indexing model from the issue text.