Updating ruby prereqs

What does this Merge Request do?

Alternate approach to !848 (closed)

Adds separate section at top of installation guide for Ruby installation. Platform and system agnostic, just links to the various version managers and collects the notes that were in other places.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.

Closes #700 (closed)