Add multi-file configuration example

**Is your feature request related to a problem? Please describe.**
For users who want to use separate configuration files for different sections of an application, an example of using multiple properties files would be helpful. Likely implement in the [andhow-sample](https://github.com/eeverman/andhow-samples/tree/main)s project main branch.

@alex-kar Can you describe a good scenario that would be a good basis for this? The more 'real' it is, hopefully the more it will resonate w/ a potential user.