docs: upgrade to docusaurus 3.9 (#623)

## 📝 Description
Upgrade Docusaurus to 3.9. This will allow us to keep up with Docusaurus
for our docs and take advantage of new features.

Changes:
- Updated docusaurus to 3.9 on packages.json
- Updated docusuaurus.config.js options

Fixes: #341

## ✅ Checklist
- [X] I have tested this change
- [ ] This change requires documentation update

Author: TomFern

docs/docusaurus.config.js

@@ -23,6 +23,10 @@ const config = {
   ],
   markdown: {
     mermaid: true,
+    hooks: {
+      onBrokenMarkdownLinks: 'warn',
+      onBrokenMarkdownImages: 'throw',
+    }
   },
 
   // Production url of your site here
@@ -37,7 +41,6 @@ const config = {
   projectName: 'semaphore',
 
   onBrokenLinks: 'throw',
-  onBrokenMarkdownLinks: 'warn',
 
   i18n: {
     defaultLocale: 'en',