Adding cloud provider oke

What does this MR do?

Include Oracle Container Engine for Kubernetes (OKE) into GitLab Chart installation documentation.

• Add Oracle K8s service URL in the index.md page, reflecting official Oracle K8s service name.
• Created Initial documentation to prepare OKE for GitLab Chart installation.

Related issues

#215701

Related to MR #1848

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened