Merge branch 'ap-add-unicorn-redirect' into 'master'

Redirect Unicorn docs to Puma

See merge request gitlab-org/gitlab!62554

doc/administration/operations/unicorn.md

+---
+redirect_to: 'puma.md'
+remove_date: '2021-08-26'
+---
+
+This file was moved to [another location](puma.md).
+
+<!-- This redirect file can be deleted after <2021-08-26>. -->
+<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->