Migrate documentation from VitePress to Docusaurus

**Motivation**
We are migrating our documentation framework from VitePress to Docusaurus to improve long-term maintainability and user experience. The decision is motivated by several factors:

- Built-in i18n support: Docusaurus provides out-of-the-box multilingual capabilities, which is essential for our audience.
- Versioned docs: Managing different versions of the documentation is significantly easier.
- Better developer ecosystem: Richer plugin system, larger community, and easier customization for landing pages, search, and analytics.
- Improved GitHub Pages integration: Docusaurus supports clean and robust CI workflows with GitHub Actions.

This issue tracks the complete migration process, including content restructuring, configuration, and CI pipeline setup.