Allow sidebar navs with custom sidebar content

Multiple sidebar navs can also be included on a page:

```
sidebar:
  - title: Navigation 1
    nav: nav1
  - title: Navigation 2
    nav: nav2
```

Fixes #1986

_includes/sidebar.html

@@ -14,6 +14,7 @@
       {% endif %}
       {% if s.title %}<h3>{{ s.title }}</h3>{% endif %}
       {% if s.text %}{{ s.text | markdownify }}{% endif %}
+      {% if s.nav %}{% include nav_list nav=s.nav %}{% endif %}
     {% endfor %}
     {% if page.sidebar.nav %}
       {% include nav_list nav=page.sidebar.nav %}