Add an advanced example (how to use hooks to load external credentials) to the configuration documentation

[stichbury]

Taken from a discussion thread on Slack

https://docs.kedro.org/en/stable/hooks/common_use_cases.html#use-hooks-to-load-external-credentials was added to the documentation but hasn't been clearly indicated.

It seems to make sense to add it to the configuration documentation as an advanced example.