Documentation: Set up test environment

[migueldivo]

Docs page: https://juju.is/docs/juju/set-up--tear-down-your-test-environment

Docs source: https://discourse.charmhub.io/t/set-up-tear-down-your-test-environment/13403

Issue: Automatic set up does not work 100% of the time.

This is because I had to restart my laptop in order for my RAM and cache to clear up. There should be a note/warning at the start to inform new users that its recommended that they do a quick restart of their laptop so that it can run properly without crashing/timing out in their first go. My experience of the whole set up would have been quicker and easier if I had known that I had to restart my laptop before doing the set up