Documentation for .ruby-variant

fullstaq-rbenv recently gained a new feature: .ruby-variant: https://github.com/fullstaq-labs/fullstaq-rbenv/issues/4
The documentation for that should be incorporated back to fullstaq-ruby-server-edition.

Start by reading fullstaq-rbenv issue #4 to understand the .ruby-variant feature, then inspect this repository’s existing documentation for the relevant Ruby configuration or installation workflow. Done means the feature is documented in the appropriate server-edition documentation and matches the behavior described in issue #4.

Written by the indexing model from the issue text.