docs: Remove /cache from ESLint example, mention cache in separate section

Created by: mrnugget

Using the /cache directory when using Docker for Mac slows down the actions by a lot. When removing the /cache the execution time for the ESLint campaign when from 16min+ to 9min in a small repository (codeintellify).

I suspect the reason for that is Docker for Mac's slow file syncing between the container and the host.

So I decided to remove the mention of cacheDirs from the example campaign and move it to its own section where it's still available for "advanced users" that want to optimize their campaign run time.