Refactor CMS to handle ContentLayer and WordPress platforms

This commit refactors the CMS to handle two platforms: ContentLayer and WordPress. The CMS layer is abstracted into a core package, and separate implementations for each platform are created. This change allows the app to switch the CMS type based on environment variable, which can improve the flexibility of content management. It also updates several functions in the `server-sitemap.xml` route to accommodate these changes and generate sitemaps based on the CMS client. Further, documentation content and posts have been relocated to align with the new structure. Notably, this refactor is a comprehensive update to the way the CMS is structured and managed.

packages/cms/contentlayer/content/docs/002-authentication/index.mdx

@@ -0,0 +1,23 @@
+---
+title: Authentication
+label: Authentication
+description: Learn everything about Authentication in Makerkit
+---
+
+MakerKit uses Supabase to manage authentication within your application.
+
+By default, every kit comes with the following built-in authentication methods:
+- **Email/Password** - we added, by default, the traditional way of signing in
+- **Third Party Providers** - we also added by default Google Auth sign-in
+- **Email Links**
+- **Phone Number**
+
+You're free to add (or remove) any of the methods supported by Supabase's
+Authentication: we will see how.
+
+This documentation will help you with the following:
+ - **Setup** - setting up your Supabase project
+ - **SSR** - use SSR to persist your users' authentication, adding new
+providers
+ - **Customization** - an overview of how MakerKit works so that you can adapt
+it to your own application's needs