Add GitLab Duo usage guidelines for community contributors

What does this MR do and why?

Adds written usage guidelines for the complimentary GitLab Duo / Duo Agent Platform access provided to community fork members.

On 2026-09-11 a per-user monthly GitLab Credits cap was applied to the gitlab-community group after a small number of members consumed the large majority of the shared AI allocation. Until now nothing written explained what the access is for, what the limits are, or how to ask for more.

The guidelines (docs/duo-usage-guidelines.md) is the single source of truth for:

• What the access is for and not for, with examples (intent-based: in service of a GitLab contribution).
• The current per-user cap, what happens when it is reached, and how to view your own usage.
• How to request a temporary increase.
• Expectations for AI-assisted contributions (you are the author; disclosure recommended, not required).
• What happens if usage falls outside the guidelines.

The GitLab Acceptable Use Policy remains the governing policy; this document links to it rather than restating it, per Legal's feedback.

Related

• gitlab-org/developer-relations/contributor-success/team-task#1326 (confidential)
• gitlab-org/developer-relations/contributor-success/team-task#1364 (confidential)
• gitlab-org/developer-relations/contributor-success/contributors-gitlab-com#703 (confidential)