docs: update MediumPost docs (#545)

* docs: update MediumPost docs

* fix: remove extra option definition

packages/example/src/pages/components/MediumPosts.mdx

@@ -9,6 +9,21 @@ Using a Gatsby plugin, this component automatically builds three `ArticleCards`
 
 </PageDescription>
 
+## Setup
+
+To update the source account for this component, pass in a `mediumAccount` to your theme options in `gatsby-config.js`.
+
+```js
+plugins: [
+  {
+    resolve: 'gatsby-theme-carbon',
+    options: {
+      mediumAccount: 'carbondesign',
+    },
+  },
+];
+```
+
 ## Example
 
 <MediumPosts color="dark" posts={3} />

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

@@ -157,15 +157,14 @@ To add a link to the bottom of each page that points to the current page source
 
 ## Medium
 
-Automatically grab posts from Medium's RSS feed and populate `ArticleCards`. Enable `gatsby-source-medium-feed` by shadowing this plugin and providing a `userName`. For guidance on how to use this plugin checkout [MediumPosts](/components/MediumPosts).
+In order to change the source account for the [`MediumPosts`](/components/MediumPosts) component, provide an account with the `mediumAccount` option.
 
 ```js
 plugins: [
   {
-    resolve: 'gatsby-source-medium-feed',
+    resolve: 'gatsby-theme-carbon',
     options: {
-      userName: '@...', // Medium user name or publication
-      name: 'MediumFeed', // GraphQL query AllMediumFeed
+      mediumAccount: 'carbondesign',
     },
   },
 ];
@@ -174,7 +173,7 @@ plugins: [
 ## Other options
 
 - `additionalFontWeights` – add support for additional Plex font weights. Don't forget to specify italics for the additional weights if needed.
-- `mdxExtensions` – change the file extensions processed by `gatsby-mdx` (default ['.mdx', '.md'])
+- `mdxExtensions` – change the file extensions processed by `gatsby-mdx` (default ['.mdx', '.md']).
 - `titleType` – pick between four formats for the `<title>` element for your site. Here are the four options using this page as an example:
 
   - `page`: "Configuration" (default)