Draft: docs: update GitLab Duo licensing requirements
Relates to issue #557386 (closed)
This MR was created by Duo in Session 1126010
Changes
- Update GitLab.com AI gateway documentation to clarify that GitLab Duo Core, Pro, and Enterprise are all supported
- Correct licensing requirements to reflect current GitLab Duo add-on tiers
- Improve clarity around available GitLab Duo add-ons for self-hosted configurations