[docs] Instructions for using external database (not use CloudNativePG)

[pvannierop]

          I think it's good to instead of deleting these values to move them to a file in the mod directory and and documentation on how to use the RADAR-Kubernetes stack while using a managed PostgreSQL server such as RDS.

Originally posted by @keyvaann in #354 (comment)

[pvannierop]

Addressed in #354