[Docs] Validate user supplied secrets or config maps don't use reserved names

We will document that users should  use a postfix like -custom for user supplied secrets. We should validate that reserved names are not used.

eg: <crName>-credentials-secret is reserved. We shold warn if that is configured in the CR - it should be something different like <crName>-credentials-secret-custom

The CR validation condition can be used to do the necessary checks