chore(docs): set addon-docs root url to '' for custom domain

Author: jamescdavis

config/addon-docs.js

@@ -6,4 +6,7 @@ const AddonDocsConfig = require('ember-cli-addon-docs/lib/config');
 module.exports = class extends AddonDocsConfig {
   // See https://ember-learn.github.io/ember-cli-addon-docs/docs/deploying
   // for details on configuration you can override here.
+  getRootURL() {
+    return '';
+  }
 };