feat: update models.yml with model descriptions

What does this merge request do and why?

Updates model metadata in models.yml to include model descriptions. These will be surfaced in the UI as a part of this work.

Descriptions have been vendor approved.

Issue: gitlab-org/gitlab#572469

Design: gitlab-org/gitlab#578569

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.