expand sidebar groups by default, remove stale plans reference

docs/content/docs/api-reference/meta.json

@@ -1,4 +1,5 @@
 {
   "title": "API Reference",
+  "defaultOpen": true,
   "pages": ["unknown"]
 }

docs/content/docs/developer/meta.json

@@ -1,5 +1,6 @@
 {
   "title": "Developer",
+  "defaultOpen": true,
   "pages": [
     "setup",
     "architecture",

docs/content/docs/meta.json

@@ -1,9 +1,4 @@
 {
   "title": "Voicebox Documentation",
-  "pages": [
-    "overview",
-    "api-reference",
-    "developer",
-    "plans"
-  ]
+  "pages": ["overview", "api-reference", "developer"]
 }

docs/content/docs/overview/meta.json

@@ -1,5 +1,6 @@
 {
   "title": "Overview",
+  "defaultOpen": true,
   "pages": [
     "introduction",
     "installation",