Site Editor: Update the new templates dropdown list (#27235)

Update the new template dropdown list to only show a hand-picked list of templates — if they are missing from the current site/theme.
Also hide the new template button if there are no more missing templates to be added.

packages/edit-site/src/components/navigation-sidebar/navigation-panel/constants.js

@@ -11,6 +11,16 @@ export const TEMPLATES_POSTS = [ 'home', 'single' ];
 
 export const TEMPLATES_STATUSES = [ 'publish', 'draft', 'auto-draft' ];
 
+export const TEMPLATES_NEW_OPTIONS = [
+	'front-page',
+	'single-post',
+	'page',
+	'archive',
+	'search',
+	'404',
+	'index',
+];
+
 export const MENU_ROOT = 'root';
 export const MENU_CONTENT_CATEGORIES = 'content-categories';
 export const MENU_CONTENT_PAGES = 'content-pages';

packages/edit-site/src/components/navigation-sidebar/navigation-panel/new-template-dropdown.js

@@ -20,7 +20,7 @@ import { Icon, plus } from '@wordpress/icons';
  * Internal dependencies
  */
 import getClosestAvailableTemplate from '../../../utils/get-closest-available-template';
-import { TEMPLATES_STATUSES } from './constants';
+import { TEMPLATES_NEW_OPTIONS, TEMPLATES_STATUSES } from './constants';
 
 export default function NewTemplateDropdown() {
 	const { defaultTemplateTypes, templates } = useSelect( ( select ) => {
@@ -59,9 +59,15 @@ export default function NewTemplateDropdown() {
 
 	const missingTemplates = filter(
 		defaultTemplateTypes,
-		( template ) => ! includes( existingTemplateSlugs, template.slug )
+		( template ) =>
+			includes( TEMPLATES_NEW_OPTIONS, template.slug ) &&
+			! includes( existingTemplateSlugs, template.slug )
 	);
 
+	if ( ! missingTemplates.length ) {
+		return null;
+	}
+
 	return (
 		<DropdownMenu
 			className="edit-site-navigation-panel__new-template-dropdown"