Fix MDX artifacts by configuring Docusaurus to process .md as MDX

Instead of renaming files to .mdx, configure Docusaurus to treat all
.md files as MDX. This is cleaner because:
- No file renames needed
- .md files render nicely in GitLab/GitHub previews
- Single config change fixes all current and future cases

Changed markdown.format from 'detect' to 'mdx' in docusaurus.config.js.

Reverted all previous .md -> .mdx renames (10 files).

Author: claude

docs/database-lab/index.mdx docs/database-lab/index.mddocs/database-lab/index.mdx renamed to docs/database-lab/index.md

@@ -40,9 +40,9 @@ module.exports = {
   organizationName: 'postgres-ai',
   projectName: 'docs',
 
-  // Use CommonMark for .md files to avoid MDX parsing issues
+  // Treat all .md files as MDX to support imports and JSX
   markdown: {
-    format: 'detect',
+    format: 'mdx',
   },
 
   customFields: {