fix(validation): allow non-object params to remark/rehype plugins (#6977)

Remark and Rehype plugins allow having options as a non-object type,
such as a string.

For instance, the official MDX docs even have an example of this:
See https://mdxjs.com/docs/extending-mdx/#using-plugins

The official plugin `remarkjs/remark-frontmatter` allows passing
a string, e.g. `"toml"` as the options arg, instead of an object.

packages/docusaurus-utils-validation/src/validationSchemas.ts

@@ -19,7 +19,7 @@ export const PluginIdSchema = Joi.string()
 
 const MarkdownPluginsSchema = Joi.array()
   .items(
-    Joi.array().ordered(Joi.function().required(), Joi.object().required()),
+    Joi.array().ordered(Joi.function().required(), Joi.any().required()),
     Joi.function(),
     Joi.object(),
   )