refactor: unify how validateOptions is handled (#6961)

* refactor: unify how validateOptions is handled

* fix types

* fix again

packages/docusaurus-plugin-content-pages/src/frontMatter.ts

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {
+  Joi,
+  validateFrontMatter,
+  FrontMatterTOCHeadingLevels,
+} from '@docusaurus/utils-validation';
+import type {FrontMatter} from '@docusaurus/plugin-content-pages';
+
+const PageFrontMatterSchema = Joi.object<FrontMatter>({
+  title: Joi.string(),
+  description: Joi.string(),
+  wrapperClassName: Joi.string(),
+  hide_table_of_contents: Joi.boolean(),
+  ...FrontMatterTOCHeadingLevels,
+});
+
+export function validatePageFrontMatter(
+  frontMatter: Record<string, unknown>,
+): FrontMatter {
+  return validateFrontMatter(frontMatter, PageFrontMatterSchema);
+}