Define a styleguide.md for Gemini Code Assist Bot

[Maijin]

Summary

Gemini Code Assist for GitHub is great but it does not follow the styleguide (flake8 etc.) of the project which introduce unwanted suggestions.

Motivation

Improve quality of AI code review. Reduce noise

Describe alternatives you've considered

N/A

Additional context

Documentation:

• https://docs.cloud.google.com/gemini/docs/code-review/style-guide
• https://docs.cloud.google.com/gemini/docs/code-review/customize-repo-review