Add docs for praefect rake tasks

doc/administration/raketasks/praefect.md

+# Praefect Rake Tasks
+
+> [Introduced]( https://gitlab.com/gitlab-org/gitlab/-/merge_requests/28369) in GitLab 12.10.
+
+## Replica checksums
+
+Prints out checksums of the repository of a given project_id on the primary as well as secondary internal Gitaly nodes.
+
+NOTE: **Note:**
+This only is relevant and works for projects that have been created on a praefect storage. See the [Praefect Documentation](../gitaly/praefect.md) for configuring Praefect.
+
+**Omnibus Installation**
+
+```shell
+sudo gitlab-rake "gitlab:praefect:replicas[project_id]"
+```
+
+**Source Installation**
+
+```shell
+sudo -u git -H bundle exec rake "gitlab:praefect:replicas[project_id]" RAILS_ENV=production
+```