Update Duo Workflow docs for private beta

GitLab Duo Workflow is moving to Beta for 17.10. To complement new Duo Workflow best practices content, we should update the existing guide.

• Start this issue's title with Docs: or Docs feedback:.

Problem to solve

Make room for new Duo Workflow best practices content.

• The current guide is a single, monolithic guide. There isn't a cohesive logical structure to the headings and content order.
• Adjust content so that each page matches TW topic types. Each page should match a single topic type. This helps with usability and SEO.
• Reorganize information architecture. Synthesize with new Duo Workflow best practices content.

Further details

N/A

Proposal

1. Migrate existing content from _index.md into individual topic types. CRCT
2. Add net new content from Duo workflow best practices enablement (#518677 - closed).
3. Adjust existing titles and links so that they match style guidelines for titles.

Who can address the issue

TW team; PM and engineering.

Other links/references

#518677 (closed)