Document how a Configurable instance loads a config file

[metaperl]

In the docs we read;

After this configuration file is loaded,

but

1. it is not clear how a configuration file for a Configurable instance is loaded... does it have to have a certain name? Does it need to be in a certain directory?
2. There is no complete working example that one can download and see the process in action

[metaperl]

This should help - https://github.com/ipython/traitlets/blob/main/examples/myapp.py