Improve Debian instructions

What does this Merge Request do?

Provide instructions for Debian that allow for choice of using asdf or not.

Closes: #1056 (closed).

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.