Website: remove deprecated onBrokenMarkdownLinks config option

Removed the deprecated top-level onBrokenMarkdownLinks configuration
option from docusaurus.config.ts. The new format using
markdown.hooks.onBrokenMarkdownLinks is already in place, making the
old option redundant. This eliminates deprecation warnings and ensures
compatibility with Docusaurus v4.

Author: dannywillems

website/docusaurus.config.ts

@@ -26,7 +26,6 @@ const config: Config = {
   projectName: 'mina-rust', // Usually your repo name.
 
   onBrokenLinks: 'throw', // Throw error on broken links to enforce link integrity
-  onBrokenMarkdownLinks: 'throw',
   onBrokenAnchors: 'warn',
 
   // Markdown configuration