Add Greek translation for Day 2

[feirw]

Summary

• Added a Greek translation of Day 2 (Variables, Built-in Functions) in Greek/02_Day_Variables_builtin_functions/.
• Included the example script variables.py.
• Linked Day 2 from the Greek README.

Test plan

• Open the Greek Day 2 markdown in preview and check images and links.
• Confirm Day 1 in Greek/readme.md navigates to the Greek Day 2 lesson.