Support deeper nesting in left sidebar

For #766 (closed): Support up to 9 levels of nesting in the left nav.

Screenshots

Generated with this nav.json:

{
  "title": "Typography",
  "path": "typography",
+ "items": [
+     {
+       "title": "Overview",
+       "path": "path-a"
+     },
+     {
+       "title": "Typescales",
+       "path": "path-b"
+     },
+     {
+       "title": "Text elements",
+       "path": "path-c"
+     },
+   ]
}

---

Shout-out to @pslaughter for pairing on this me as part of #donut-be-silos-fe 😊