Doc - Add flavor switch

[SimonMellerin]

Migration to flavored documentation

• [x] index.md
• [ ] configuration
  • [ ] reference.md
• [x] backup_restore.md
• [ ] configuration.md
• [x] contribute
  • [x] contribute.md
  • [x] pack.md
  • [x] guide.md
• [x] getting-started
  • [x] database-vendors.md
  • [x] installation.md
  • [x] introduction.md
  • [x] basics.md
• [x] stats.md
• [x] anonymization
  • [x] workflow.md
  • [x] custom-anonymizers.md
  • [x] essentials.md
  • [x] performance.md
  • [x] internals.md
  • [x] packs.md
  • [x] core-anonymizers.md
  • [x] doctrine-inheritance.md
  • [x] command.md