Need Documentation for Cleaning Up Nexus Usage

## What New Or Updated Would You Like To See?

We would like to be able to advise customers the best way to configure their Nexus repositories, including guidance on how to set up cleaning up old (unrequired) packages, with enough guidance to help the user understand when they shouldn't clean stuff up (e.g. build dependencies)

## Why Is It Needed?

Folk ask about this a few times a year.

## Additional Context?

I initially proposed that we could add this to the [environment deployment](https://github.com/chocolatey/choco-quickstart-scripts/issues/252), but it was decided that we should just have this as docs.