[blog] Fix missing card

See how https://twitter.com/MUI_hq/status/1688919247504957440 doesn't have a card.
It's a follow-up on #38258.

I think that we should be able to fail the build if the card is missing, but too lazy.

docs/pages/blog/discord-announcement.md

@@ -7,7 +7,7 @@ tags: ['News']
 card: true
 ---
 
-<a href="https://mui.com/r/discord/"><img src="/static/blog/2023-discord-announcement/discord.png" width="1280" height="640" style="margin-bottom: 16px;" alt="Discord banner with link" /></a>
+<a href="https://mui.com/r/discord/"><img src="/static/blog/discord-announcement/discord.png" width="1280" height="640" style="margin-bottom: 16px;" alt="Discord banner with link" /></a>
 
 This year, one of our highest priorities has been to create a central hub for the MUI community.
 Up until now, our community has been scattered across discussions and interactions on [Twitter](https://twitter.com/MUI_hq), [LinkedIn](https://linkedin.com/company/mui), [GitHub](https://github.com/mui), and [Stack Overflow](https://stackoverflow.com/questions/tagged/material-ui).

packages/markdown/parseMarkdown.js

@@ -1,4 +1,6 @@
 const { marked } = require('marked');
+const fs = require('fs');
+const path = require('path');
 const kebabCase = require('lodash/kebabCase');
 const textToHash = require('./textToHash');
 const prism = require('./prism');
@@ -675,6 +677,20 @@ ${headers.hooks
       };
     });
 
+  if (docs.en.headers.card) {
+    const slug = docs.en.location.replace(/\/docs\/pages\/blog\/(.*)\.md/, '$1');
+    const exists = fs.existsSync(
+      path.resolve(__dirname, `../../docs/public/static/blog/${slug}/card.png`),
+    );
+
+    if (!exists) {
+      throw new Error([
+        `MUI: the card image for the blog post "${slug}" is missing.`,
+        `Add a docs/public/static/blog/${slug}/card.png file then restart Next.js or remove card: true from the headers.`,
+      ].join('\n'));
+    }
+  }
+
   return { demos, docs };
 }