feat: sub-categories in sidebar.json (#892)

* Add ability to specify sub categories in sidebar.json

* move v1 files to correct place

* refactor implementation

* refactor naming for implementation clarity

v1/lib/server/utils.js

@@ -66,10 +66,30 @@ function autoPrefixCss(cssContent) {
     .then(result => result.css);
 }
 
+// Validate the docs in the sidebar are valid
+function validateSidebar(metadata, sidebarMetadatas) {
+  if (metadata.next) {
+    if (!sidebarMetadatas[metadata.next]) {
+      throw new Error(
+        metadata.version
+          ? `Improper sidebars file for version ${
+              metadata.version
+            }, document with id '${
+              metadata.next
+            }' not found. Make sure that all documents with ids specified in this version's sidebar file exist and that no ids are repeated.`
+          : `Improper sidebars.json file, document with id '${
+              metadata.next
+            }' not found. Make sure that documents with the ids specified in sidebars.json exist and that no ids are repeated.`,
+      );
+    }
+  }
+}
+
 module.exports = {
   getSubDir,
   getLanguage,
   isSeparateCss,
   minifyCss,
   autoPrefixCss,
+  validateSidebar,
 };