Extend ways to configure the core application

### Description

Currently, the only way to configure the core application is [through `flowforge.yml` file](https://flowfuse.com/docs/install/configuration/).
As an application operator, I would like to provide multiple configuration files or use environmental variables to configure a single parameter.

Real-life example - instead of keeping the database password in yaml file I would prefer to store it in Vault and inject it into a VM or Kubernetes pod using environmental variables.

### Have you provided an initial effort estimate for this issue?

I can not provide an initial effort estimate

Start with the configuration documentation and the current flowforge.yml handling described in the issue. Define how multiple configuration files and environment variables should configure a single parameter, including the database password example; the work is done when both configuration methods are supported for the core application.

Written by the indexing model from the issue text.