Add GenAI Contribution Policy for Cortex contributors

[alolita]

Contributors in almost all open source projects are starting to use Generative AI such as LLMs and other generative tools to participate in issues and create pull requests.

OpenTelemetry, the second most popular cloud-native project has published a Generative AI Contribution Policy for all contributors. See https://github.com/open-telemetry/community/blob/main/policies/genai.md and https://github.com/open-telemetry/opentelemetry-go-compile-instrumentation/blob/main/AI_POLICY.md

Cortex needs to have a similar policy for using GenAI for contributions.