[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).

docs/pages/blog/lab-tree-view-to-mui-x.md

@@ -4,7 +4,7 @@ description: Migrate to the new package to start building with our powerful Tree
 date: 2023-08-21T00:00:00.000Z
 authors: ['flaviendelangle']
 tags: ['MUI X', 'News']
-card: true
+card: false
 ---
 
 After more than 4 years in the lab, the [Tree View](https://mui.com/x/react-tree-view/) components have found a new home as part of MUI X.

docs/pages/experiments/blog/blog.md

@@ -3,7 +3,7 @@ title: Blog post title
 description: Our internationally distributed startup gathered on a remote island to get to know each other better. Here's what happened!
 date: 2022-07-28T00:00:00.000Z
 authors: ['samuelsycamore']
-card: true
+card: false
 tags: ['Company']
 ---
 

docs/src/modules/components/TopLayoutBlog.js

@@ -247,7 +247,7 @@ function TopLayoutBlog(props) {
   const { description, rendered, title, headers } = docs.en;
   const finalTitle = title || headers.title;
   const router = useRouter();
-  const slug = router.pathname.replace(/\/blog\//, '');
+  const slug = router.pathname.replace(/(.*)\/(.*)/, '$2');
   const { canonicalAsServer } = pathnameToLanguage(router.asPath);
   const card =
     headers.card === 'true'

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 +677,22 @@ ${headers.hooks
       };
     });
 
+  if (docs.en.headers.card === 'true') {
+    const slug = docs.en.location.replace(/(.*)\/(.*)\.md/, '$2');
+    const exists = fs.existsSync(
+      path.resolve(__dirname, `../../docs/public/static/blog/${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 };
 }