Plugins added back to values.yaml

General MR

Summary

Some plugins and grafana settings were removed from the grafana.ini section of values.yaml in a previous chart update (looks like from 6.58 -> 6.60). These were causing the Redis datasources to not be found and may have had other consequences.

Also added a DEVELOPMENT_MAINTENANCE.md since there wasn't one for this package and wanted a place to document the Bigbang deviations from upstream helm.

Relevant logs/screenshots

N/A