fix: remove extraLayouts

This is not longer supported as you can define your own layouts as components,
following this guide: https://www.gatsbyjs.com/docs/how-to/routing/layout-components/

packages/example/src/pages/guides/configuration.mdx

@@ -380,27 +380,12 @@ array in the theme options.
 
 ## Adding new MDX layouts
 
-`gatsby-plugin-mdx` and `gatsby-source-filesystem` can work together to
-automatically template MDX files in a given directory. See the
-[plugin documentation](https://www.gatsbyjs.com/plugins/gatsby-plugin-mdx/#default-layouts).
+This would need to be added as custom components following this
+[Layout component docs](https://www.gatsbyjs.com/docs/how-to/routing/layout-components)
+outlined by Gatsby.
 
-The Carbon theme already comes with a default layout, but you can add your own
-with the `extraLayouts` option.
-
-```js
- plugins: [
-    {
-      resolve: 'gatsby-theme-carbon',
-      options: {
-        extraLayouts: {
-          posts: require.resolve('./src/layouts/postLayout.js')
-        }
-      },
- ],
-```
-
-Note that the extra layouts do not inherit from theme default. Custom layouts
-start from a clean slate. If you want to iterate on the Carbon template, copy
-`/gatsby-theme-carbon/src/templates/Default.js` to your custom template as a
+Note that the layouts do not inherit from theme default. Custom layouts start
+from a clean slate. If you want to iterate on the Carbon template, copy
+`/gatsby-theme-carbon/src/components/Layout.js` to your custom template as a
 starting point. When doing this, you may also need to change the import paths to
 the correct location.

packages/gatsby-theme-carbon/gatsby-config.js

@@ -34,7 +34,6 @@ module.exports = (themeOptions) => {
     gatsbyPluginSharpOptions = {},
     isServiceWorkerEnabled = false,
     isSwitcherEnabled = true,
-    extraLayouts = {},
   } = themeOptions;
 
   const theme = { ...defaultTheme, ...themeOption };
@@ -119,7 +118,6 @@ module.exports = (themeOptions) => {
           defaultLayouts: {
             default: require.resolve('./src/templates/Default.js'),
             home: require.resolve('./src/templates/Homepage.js'),
-            ...extraLayouts,
           },
         },
       },