Refactor to support custom Markdoc components (#52)

* Refactor to support custom Markdoc components

Introduced a new file for custom Markdoc components and updated the client to render Markdoc content using React components. Modified content renderer to handle React nodes instead of dangerously setting inner HTML.

* Add custom Markdoc tags support in Keystatic

Introduced `CustomMarkdocTags` for defining user tags. Updated `transform` to accommodate the new custom tags, enhancing tag customization in the CMS.

packages/cms/keystatic/src/content-renderer.tsx

@@ -1,3 +1,3 @@
 export function KeystaticContentRenderer(props: { content: unknown }) {
-  return <div dangerouslySetInnerHTML={{ __html: props.content as string }} />;
+  return <div>{props.content as React.ReactNode}</div>;
 }