Oh, sure! 
First, here’s my content architecture:
.
├── content
│ ├── dental_work
│ │ ├── pediatrics.md
│ │ ├── young-adult.md
│ │ ├── etc-etc.md
│ ├── careers
│ │ ├── cities.md
│ │ ├── suburban.md
│ │ ├── etc-etc.md
│ ├── mentoring
│ │ ├── universities.md
│ │ ├── communities.md
│ │ ├── etc-etc.md
So, as I wrote earlier, If I am in page pediatrics.md, I want all the pages in the same section to be listed in a side menu, and the current page to be highlighted with class=active. .Site.Menu seems to fit this purpose.
First, each section gets a menu name. So, not just one menu called main as explained in the Hugo manual. Therefore, in pediatrics.md, the front matter is:
menu:
dental_work:
parent: "dental_work"
weight: 1
All the content pages in dental_work must have this front matter. weight is for the ordinal display in the menu.
Pages in section careers get this front matter:
menu:
careers:
parent: "careers"
weight: n
Pages in section mentoring get this front matter:
menu:
mentoring:
parent: "mentoring"
weight: n
In the sidebar navigation template that I made for the page template, I have:
{{ $currentNode := . }}
{{ $thisSection := .Section }}
{{ range (index .Site.Menus $thisSection) }}
{{ if .HasChildren }}
<ul class="nav" id="section-list">
{{ range .Children }}
<li class="{{if $currentNode.IsMenuCurrent $thisSection . }} active{{end}}"><a href="{{.URL}}">{{ .Name }}</a></li>
{{ end }}
</ul>
{{end}}
{{end}}
That’s it. Let me know if I’m missing anything.
-Z