docs: sync README/SUPPORT_MATRIX/CHANGELOG with actual repo inventory

[yeongseon]

Summary

Fixes documentation drift found in the code↔docs sync audit (#42). Docs now match what is actually on disk and in CI.

Changes

• README
  • Add existing-but-undocumented examples: quickstart/5min-sqlalchemy, templates/flask (11 recipes), templates/django, fundamentals/pycubrid (16), fundamentals/sqlalchemy (7), fundamentals/pandas (6).
  • Refresh Templates table, Fundamentals table, Framework Map, and Project Structure tree to reflect the real directory layout.
• SUPPORT_MATRIX
  • Recipe total corrected 55/58 → 62 (actual sum of the category table: 16+7+6+11+12+5+1+4).
  • Python 3.12 marked as CI default (both ci.yml and smoke-test.yml run 3.12); 3.10 reframed as supported minimum.
• CHANGELOG
  • Remove false "Multilingual README support (6 languages)" claim — only English README.md exists.

Verification

• Directory inventory cross-checked against disk.
• Version facts confirmed: Python 3.12 in CI, CUBRID 11.2 in compose/CI.
• grep confirms no lingering 55/58 recipe claims or multilingual references.

Closes #42