Add guide to prepare Ruby
Problem
GDK requires specific Ruby version to be installed, according to what is used in the gitlab
project. Installing specific Ruby versions and switching from system Ruby may not be very straightforward.
Some examples of past questions relating to Ruby installation:
Proposed solution
Improve documentation for setting up Ruby for development.
There is already a WIP MR !848 (closed). Creating this issue to track it in the GDK board.
/cc @kwiebers