[docs] Clarify Secret Management in Backend for Tauri Build

[jorgechato]

When you are working with 3rd party services like an API or a database either with a replica sync or remote database server, you need to provide the secrets or auth tokens.

The usage of env variables with dotfiles of direct env variables are well documented in dev environment.

Never the less when you build the app it's unclear how to manage this secrets. pnpm tauri build --bundles app doesn't build the app with the env secrets.
The documentation recommend the usage of stronghold but all the information I could find uses stronghold as a secret storage for user token/secrets (frontend sending this secrets to backend)..

I would like to have a documentation related to the distribution with secrets needed in the backend(src-tauri) side.