Add gitlab:db:dictionary:generate rake task

What does this MR do and why?

Adds the gitlab:db:dictionary:generate rake task and runs it after db:migrate to generate missing database dictionary docs for new tables.

Relates to gitlab-org/database-team/team-tasks#244 (closed)

How to set up and validate locally

1. Create a database migration that adds a new table
2. Run bin/rake db:migrate
3. You should have a new file db/docs/<table_name>.yml

MR acceptance checklist

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