Confidential issues redesign revert docs
Reverting the new confidential issues design, per https://gitlab.com/gitlab-org/gitlab-ce/issues/33840.
Reverting the docs here.
The docs were already reverted and merged into master. But this is the merge request to pick into stable. So there are just some small changes to make sure it makes it into the stable branch.
The code feature revert is here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12307.
Edited by Victor Wu