Improve navigation

### Problem description

Currently the documentation article contains two navigation widgets, one in the left sidebar and one in the right sidebar. The one on the left is used to jump between navigation articles and the one on the right is an outline used to navigate through the headings of the current article.


The distinction between navigation in the same article and between different articles is probably irrelevant for the user, so using distinctive UI for those concepts may be confusing.

### Expected behavior

Both widgets could be consolidated in a single widget, e.g. the sidebar on the left. The outline headings could appear below their heading in the sidebar on the left.