Move Omniauth providers under Omniauth

What does this MR do and why?

Moves the Omniauth providers under Omniauth, because they're a subset of Omniauth as per https://docs.gitlab.com/ee/integration/omniauth.html#supported-providers

Review app: https://docs-omniauth-nav.docs.gitlab-review.app/ee/integration/omniauth.html

Screenshots, screen recordings, or links to review app

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Review app: https://docs-omniauth-nav.docs.gitlab-review.app/ee/integration/omniauth.html

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 confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.