Simplified "starting pharo" instructions

[fuhrmanator]

Based on the comments on reddit for the 7.0 release, I think it would be good to have a "How to get started" documentation that's up to date and really simple. I had a look at Pharo by Example and it's pretty out of date (Pharo Launcher is very different now). It would be nice to have:

• Installing Pharo with Pharo Launcher (the curl/bash approaches are technical and set off alarms for people who are rightly paranoid of running scripts to install things).
• Up-to-date explanation of Pharo Launcher
• Explaining simply what is a Pharo environment: Image, VM, Package, File, Git repo. I'm not sure beginners will need the .changes files or more details. They're in the other docs anyway. Here's a diagram (I can attach the PlantUML source if you need) to give an idea:

[fuhrmanator]

The diagram could be updated to include important aspects from https://github.com/pharo-vcs/iceberg/wiki/Iceberg-glossary