docs: new and improve lexical docs, hoist up all headings (#6639)

docs/fields/code.mdx

@@ -21,7 +21,7 @@ keywords: code, fields, config, configuration, documentation, Content Management
 
 This field uses the `monaco-react` editor syntax highlighting.
 
-### Config
+## Config
 
 | Option             | Description                                                                                                                                                                 |
 | ------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
@@ -44,7 +44,7 @@ This field uses the `monaco-react` editor syntax highlighting.
 
 _\* An asterisk denotes that a property is required._
 
-### Admin Config
+## Admin Config
 
 In addition to the default [field admin config](/docs/fields/overview#admin-config), you can adjust the following properties:
 
@@ -53,7 +53,7 @@ In addition to the default [field admin config](/docs/fields/overview#admin-conf
 | **`language`**      | This property can be set to any language listed [here](https://github.com/microsoft/monaco-editor/tree/main/src/basic-languages).                                               |
 | **`editorOptions`** | Options that can be passed to the monaco editor, [view the full list](https://microsoft.github.io/monaco-editor/typedoc/interfaces/editor.IDiffEditorConstructionOptions.html). |
 
-### Example
+## Example
 
 `collections/ExampleCollection.ts