Documentation for LFS object storage

What

Adds documentation for use of LFS with object storage

Preview

https://gitlab.com/gitlab-org/gitlab-ee/blob/jej/lfs-object-storage-docs/doc/workflow/lfs/lfs_administration.md

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated

Related

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2760
• https://gitlab.com/gitlab-org/gitlab-ee/issues/3362