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/content-renderer.tsx

@@ -1,6 +1,7 @@
-import { DocumentElement } from '@keystatic/core';
-import { DocumentRenderer } from '@keystatic/core/renderer';
+import * as React from 'react';
 
-export function KeystaticDocumentRenderer(props: { content: unknown }) {
-  return <DocumentRenderer document={props.content as DocumentElement[]} />;
+import Markdoc from '@markdoc/markdoc';
+
+export function KeystaticDocumentRenderer({ content }: { content: unknown }) {
+  return Markdoc.renderers.react(content as string, React);
 }