docs: add testing guide and replace Russian with Ukrainian

- Add how-to/testing.md with pytest fixtures and CI/CD patterns
- Add Testing section to navigation in mkdocs.yaml
- Replace Russian language with Ukrainian in languages tutorial

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

mkdocs.yaml

@@ -86,6 +86,8 @@ nav:
           - Migrate to 2.0: how-to/migrate-to-v20.md
           - Alter Tables: how-to/alter-tables.md
           - Backup and Restore: how-to/backup-restore.md
+      - Testing:
+          - Testing Best Practices: how-to/testing.md
   - Reference:
       - reference/index.md
       - Specifications: