docs: clarifies that `duo ask` uses classic, add link to duo cli (!2804) · Merge requests · GitLab.org / cli · GitLab

Description

Updates ahead of the GitLab Duo CLI experimental release:

Clarifies that the glab ask command uses GitLab Duo (Classic)
Adds link to the new GitLab Duo CLI experimental docs (MR: GitLab Duo CLI docs for experimental release (gitlab!214579 - merged))