Add database maintenance example

[marcleblanc2]

Adding an example override file to scale pods to 0 which would try to connect to the databases during database maintenance operations, ex. MVU, and provide an easy place for customers to operate the migrator commands.

Customers can use this override file with the sourcegraph Helm chart, on top of their own override file(s)

Checklist

• Follow the manual testing process
• Update changelog
• Update Kubernetes update doc

Test plan

• Tested on test instance
• Have used with customers in the past