Refactor: Rename EXCLUDE_FORKED_REPOS to EXCLUDE_CONTRIB_REPOS

[koichincom]

The previous variable name was misleading. It doesn't just exclude forked repositories - it excludes all repositories the user has contributed to but doesn't own (whether they're forks or original projects).

Changes:

• Rename EXCLUDE_FORKED_REPOS → EXCLUDE_CONTRIB_REPOS
• Rename ignore_forked_repos → exclude_contrib_repos in Python code
• Add backward compatibility in generate_images.py with deprecation warning
• Update README.md with clearer documentation explaining the behavior

The new name accurately reflects what the variable does: exclude contributed repositories from statistics calculation.