Write "getting started" guide

This should go in https://buildstream.gitlab.io/buildstream/, probably as part of the installation guide

This should cover:

• what it's for in a couple of sentencess.
• how to install it (via a link if it's complex)
• how to do 'hello-world' within 10 minutes, with words people with no bst experience can understand
• where to dig deeper

As a reference, GNOME people wrote this to replace its jhbuild guide when they migrated to bst: https://wiki.gnome.org/Newcomers/BuildSystemComponent