Update Dev/Maintenance guide

Summary

Dev and Maintenance guides need to be reviewed/expanded.

One function that needs to be added is to check the minio-instance update and ensure that this chart is updated to reflect the same versions of opensource/minio/minio and opensource/minio/mc that were tested and validated in the subchart.

The testing section should also include more functional overrides or a reference to existing overrides to deploy mattermost with its additional functions.