There is also a widget for showing the TOC article or a category list in the sidebar. It is a “multi-instance widget” so you can have more that one version of the widget active at a time.
This works similarly to the TOC shortcode. If you just go with the defaults then the contents of the special TOC page is shown. Alternatively, you can select a wiki category and options sort order and only show articles for a particular category.
This documentation uses the TOC widget for the wiki documentation navigation.