feat: Add ruby code - chapter "Graph"

[khoaxuantu]

If this pull request (PR) is associated with coding or code transpilation, please attach the relevant console outputs to the PR and complete the following checklist:

• I have thoroughly reviewed the code, focusing on its formatting, comments, indentation, and file headers.
• I have confirmed that the code execution outputs are consistent with those produced by the reference code (Python or Java).
• The code is designed to be compatible on standard operating systems, including Windows, macOS, and Ubuntu.

[krahets]

Thanks for the clean code!