Add guide about Clickhouse migrations
Add a guide about Clickhouse migrations.
Automated execution of Clickhouse migrations will be added soon, by https://ops.gitlab.net/gitlab-com/gl-infra/deploy-tooling/-/merge_requests/484.
This MR adds a guide to Clickhouse migrations for Release managers based on the discussion in gitlab-com/gl-infra/delivery#19948 (comment 1758634868).
Edited by Reuben Pereira