Add documentation how to configure nestjs app outside app context

[BrunnerLivio]

I'm submitting a...

[ ] Regression 
[ ] Bug report
[ ] Feature request
[x] Documentation issue or request (new chapter/page)
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

At the moment it only describes how you can get the configuration using dotenv inside the context

Expected behavior

There should be a section on the techniques/configuration site on how to inject configuration outside of the app context.

What is the motivation / use case for changing the behavior?

My application should be programmatically configurable.

My application interface looks like this in the end:

import { MyApp } from '@my/app';

new MyApp({ mySettings: true });

The class MyApp creates a Nest Execution Context. The settings given to the class constructor should be able to pass to this execution context by for example NestFactory.createApplicationContext(AppModule.forRoot(settings)).