ENV concepts section should raise awareness of `file` placeholder for secrets

[polarathene]

Short of using search for the keyword secret this might not be as easily discovered? Caddy could better raise awareness of the feature for a common use-case where secrets often leak.

Since it's a common practice to pass secrets to services via ENV (which isn't ideal given various scenarios that can leak, undocumented advice from @francislavoie would be good to add too for added context with the env placeholder, similar for why one may prefer using a file placeholder to more securely acquire secrets.