Add GlStackedProgressBar documentation

Description

Update the GlProgressBar documentation with information about GlStackedProgressBar

Checklist

Make sure the following are completed before closing the issue:

1. Assign the correct component label to this issue.
2. Create an MR with the additions or updates needed.
3. When applicable, create an issue using the Figma update issue template to update the component in Figma, and mark it as related to this one. Bring the issue to your team planning session for prioritization and scheduling.
   • Figma > GlStackedProgressBar - Add to component... (#2001 - closed)
4. When applicable, create an MR in GitLab UI to update the component. If you do not have capacity or are unable to update the component directly, create a GitLab UI issue using the Component issue template. Mark the new issue as related to this one. Bring the issue to your team planning session for prioritization and scheduling. Mark the issue as related to this one.
   • feat(GlStackedProgressBar): Implement GlStacked... (gitlab-org/gitlab-ui!4987 - closed)
5. When introducing a major or breaking change, communicate the changes within the Engineering Week in Review and UX Weekly meeting.
6. 🎉 Congrats, you made it! You can now close this issue.