chore: update Docusaurus to v3.9.1 and adjust dependencies versions (#1014)

# 🚀 Upgrade Docusaurus to v3.9.1

## Summary

This PR upgrades the Tolgee documentation site from Docusaurus 3.8.1 to
3.9.0, bringing performance improvements, new features, and important
security updates.

## 📋 Changes Made

### Core Package Updates
- **Docusaurus Core**: Updated all `@docusaurus/*` packages from
`^3.8.1` to `^3.9.1`
  - `@docusaurus/babel`, `@docusaurus/core`, `@docusaurus/mdx-loader`
  - `@docusaurus/plugin-content-docs`, `@docusaurus/preset-classic`
- `@docusaurus/theme-classic`, `@docusaurus/theme-common`,
`@docusaurus/utils`
- All development dependencies including `@docusaurus/eslint-plugin`,
`@docusaurus/module-type-aliases`, etc.

### Dependency Updates
- `@typescript-eslint/eslint-plugin`: `^8.39.0` → `^8.44.1`
- `@typescript-eslint/parser`: `^8.39.0` → `^8.44.1`
- `axios`: `^1.8.1` → `^1.12.2`
- `posthog-js`: `^1.165.0` → `^1.268.5`
- `tsx`: `^4.19.3` → `^4.20.6`
- `clsx`: `^1.1.1` → `^2.1.1`

### Configuration Updates
- **Node.js Engine**: Updated minimum requirement from `>=18.0` to
`>=20.0` (required for Docusaurus 3.9)
- **Configuration Migration**: Updated `docusaurus.config.ts` to use new
`markdown.hooks.onBrokenMarkdownLinks` format instead of deprecated
`onBrokenMarkdownLinks`

## ✨ New Features Available

### 🔍 Algolia DocSearch v4 with AskAI
- Support for AI-powered search assistant that can answer questions
based on documentation content
- Opt-in feature - can be enabled by upgrading `@docsearch/react` and
configuring `themeConfig.algolia.askAi`

### 🌐 Enhanced i18n Support
- New `i18n.localeConfigs[locale].{url,baseUrl}` options for complex
multi-domain deployments
- Improved translation infrastructure with
`i18n.localeConfigs[locale].translate` flag
- Support for explicit translation keys in sidebar items to avoid
conflicts

### 📊 Mermaid ELK Layout Support
- Support for Eclipse Layout Kernel (ELK) algorithm for more
sophisticated diagram layouts
- Can be enabled by adding `@mermaid-js/layout-elk` dependency

### 🚀 Performance Improvements
- Upgraded to Rspack 1.5 with new configuration options for faster
builds
- Optimized i18n infrastructure for non-i18n sites
- Improved visual glitch handling in color mode switching

## 🔧 Technical Details

### Breaking Changes
- **Node.js 18 Support Dropped**: Minimum Node.js version is now 20.0
(Node.js 18 reached End-of-Life)
- **Configuration Migration**: `onBrokenMarkdownLinks` moved to
`markdown.hooks.onBrokenMarkdownLinks`

### Compatibility
- ✅ Current Node.js version (20.19.5) meets requirements
- ✅ All existing configurations remain compatible
- ✅ No breaking changes affecting current functionality

## 🧪 Testing

- ✅ **Build Test**: `npm run build` completes successfully with no
errors
- ✅ **Development Server**: `npm start` runs correctly on localhost:3000
- ✅ **Dependencies**: All packages installed without vulnerabilities
- ✅ **Configuration**: New configuration format validated

## 📚 References

- [Docusaurus 3.9 Release
Notes](https://docusaurus.io/blog/releases/3.9)
- [Docusaurus 3.9.0 Changelog](https://docusaurus.io/changelog/3.9.0)
- [Node.js Release Schedule](https://nodejs.org/en/about/releases/)

## 🎯 Impact

- **Performance**: Faster builds with Rspack 1.5 optimizations
- **Security**: Updated dependencies with latest security patches
- **Features**: Access to new Docusaurus 3.9 features for future
enhancements
- **Maintenance**: Aligned with latest Docusaurus release for continued
support

---

**Note**: This upgrade maintains full backward compatibility with
existing content and configurations. The site will continue to function
exactly as before, with improved performance and access to new optional
features.

Author: dzienisz

docusaurus.config.ts

@@ -16,7 +16,11 @@ const config: Config = {
   baseUrl: '/',
   onBrokenLinks: 'throw',
   favicon: 'img/favicon.svg',
-  onBrokenMarkdownLinks: 'warn',
+  markdown: {
+    hooks: {
+      onBrokenMarkdownLinks: 'warn',
+    },
+  },
   organizationName: 'Tolgee',
   projectName: 'Tolgee',
   themes: ['docusaurus-theme-openapi-docs'],