chore: Brush up of PowerDNS Documentation Site

chore: Brush up of PowerDNS Documentation Site

- Upgraded to Bootstrap 5.3.3 for improved styling and responsiveness.
- Redesigned the documentation site to align with the visual style of other PowerDNS sites.
- Enhanced the version selector functionality:
  - Fetches and displays available versions from a `versions.json` file.
  - Supports additional and legacy versions with a dropdown for more options.
  - Highlights the latest version and provides a fallback for legacy versions if fetching fails.
- Improved user interface with a modern look and feel, including updated navigation, hero section, and product cards.