chore: Duo UI update docs

What does this MR do?

This MR introduces the first iteration of the Duo-UI documentation after forking from GitLab-UI. We have removed tools and topics that will not be migrated to Duo-UI, and updated the documentation for those that will remain a part of Duo-UI.

Some updates introduced in this MR still need to be applied to the main branch, and we are working on that. We’re taking a docs-first approach for this iteration.