Merge branch 'docs-db-downtime-redirect' into 'master'

Add redirect for renamed doc

See merge request gitlab-org/gitlab!58728

doc/development/what_requires_downtime.md

+---
+redirect_to: 'avoiding_downtime_in_migrations.md'
+---
+
+This document was moved to [another location](avoiding_downtime_in_migrations.md).
+
+<!-- This redirect file can be deleted after <2021-07-01>. -->
+<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->