Add instructions to keep ruby plugin up-to-date in rtx

What does this merge request do and why?

This MR updates the https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/rtx.md page to add instructions on how to keep the asdf-ruby plugin up-to-date when using rtx. This avoids situations where a new ruby version is pulled in .tool-versions but the locally installed plugin is not aware of the new Ruby version.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.