Delete index.erb

What does this MR do and why?

Describe why, not just what. Include links to relevant issues or discussions.

Screenshots, screen recordings, or links to review app

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How to set up and validate locally

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

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

Merge request acceptance checklist

This checklist encourages us to maintenancerefactor