Document use case of splitting auto-configuration and starter

[snicoll]

In the "Creating your own starter" section we explain the "full" infrastructure for a starter: an auto-configuration module and a separate "starter pom" that brings the auto-configuration and the dependencies that are needed to get started.

We also mention that those two can be merged in a single artifact but we don't really explain the rationale behind the separate artifacts, i.e. that It starts to make sense to split things up if you want to provide auto-configuration for various combinations of dependencies.

[snicoll]

Someone asked a related question and I believe this answer provides some context we could reuse.