Integrate Markdoc library with Keystatic CMS.

The commit introduces the '@markdoc/markdoc' library to the Keystatic CMS. It modifies related functions and components such as 'KeystaticDocumentRenderer' and 'keystatic-client' to transform and render content using Markdoc. Additionally, the commit refactors 'keystatic.config.ts' file for more legible content field setup and adds related dependencies in the package.

packages/cms/keystatic/src/keystatic-client.ts

@@ -1,3 +1,5 @@
+import Markdoc from '@markdoc/markdoc';
+
 import { Cms, CmsClient } from '@kit/cms';
 
 import { createKeystaticReader } from './create-reader';
@@ -138,7 +140,8 @@ class KeystaticClient implements CmsClient {
       ? new Date(item.entry.publishedAt)
       : new Date();
 
-    const content = await item.entry.content();
+    const markdoc = await item.entry.content();
+    const content = Markdoc.transform(markdoc.node);
 
     return {
       id: item.slug,