Document how to keep Praefect's SQL schema up to date

Praefect has its own SQL database. Every time you deploy a new version of Praefect you need to make sure you run any pending SQL migrations.

The status quo in multi-node GitLab installations is that the local administrators must have their own process for running database migrations. In the gitlab-rails case this means running rake db:migrate. https://docs.gitlab.com/omnibus/update/#zero-downtime-updates

I think we should extend this documentation with information on how to run Praefect migrations.