add reminder about AI policy

[maryliag]

Add ourAI policy as one of the reminder for contributors

[Copilot]

Pull request overview

This PR adds a reminder about the OpenTelemetry Generative AI (GenAI) policy to the first-time contributor welcome message. It fits into the first-time-pr reusable workflow that auto-comments on first-time contributors' PRs, and updates its accompanying documentation to match.

Changes:

• Added a bullet linking to the OpenTelemetry GenAI policy (disclose AI use, communicate in your own words) in the workflow's generated welcome comment.
• Mirrored the same bullet in the README's documented "Default comment body".

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/first-time-pr.yml	Adds the GenAI policy reminder bullet to the actual welcome comment posted on first-time PRs.
first-time-pr/README.md	Adds the same GenAI policy reminder bullet to the documented default comment body, keeping docs in sync.

I verified that the linked policy URL (open-telemetry/community/blob/main/policies/genai.md) exists and that its content matches the summary in the reminder text. Both files containing the reminders list were updated consistently, the new line matches the formatting of surrounding bullets, and I found no spelling, grammar, or correctness issues.

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