Sidebar navigation
The left sidebar nav tree is built from site.Menus.sidebar — falling back to site.Menus.main if no sidebar menu is defined — rendered by sidebar-nav.html. This site’s own sidebar (see the left of this page) is built exactly this way; its hugo.toml defines [[menus.sidebar]] entries with identifier/parent/weight to create the grouped, collapsible tree you’re looking at.
A top-level entry with an empty url = '' and an identifier renders as an expandable group header; entries that set parent = "<identifier>" nest underneath it.
[[menus.sidebar]]
name = 'Get started'
identifier = 'get-started'
url = ''
weight = 10
[[menus.sidebar]]
name = 'Install'
parent = 'get-started'
url = '/get-started/install/'
weight = 10sidebar_nav_title sets the heading shown above the tree (defaults to site.Title if unset).
Not yet implemented
Upstream’s show_nav_level, navigation_depth, and collapse_navigation options are declared in this module’s hugo.toml defaults (carried over from the Sphinx theme.conf) but have no template logic behind them yet — the sidebar tree always renders fully expanded. See Feature parity with upstream
.