Document config-db default retention period

[adityathebe]

const (
	DefaultConfigAnalysisRetentionDays = 60
	DefaultConfigChangeRetentionDays   = 60
	DefaultConfigItemRetentionDays     = 7
)

github.com/flanksource/config-db/jobs/cleanup.go

Use this as a reference and document that in /docs/guide/config-db/concepts/retention