chore: remove old root-level content and GitBook artifacts

All content already migrated to docs/ for MkDocs.
Removes duplicate root files, .gitbook/, SUMMARY.md, and root CNAME.

Repo structure is now:
  docs/         ← EN content + es/, hu/ translations
  mkdocs.yml
  README.md
  .github/

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: autoblitzbot