Docs: updates to code review flow page

What does this MR do?

Iterates on the docs updates from Docs: Add Code Review as Foundational Flow for DAP (!214587 - merged)

Summary of changes

• Applies style guide edits
• Ensures all mentions of Duo have GitLab in front of them for legal reasons
• Reworks the introduction to match the format and style of other Flows. I used Duo to help pull together the list.
• Adds a one-line note to each of the code review pages to mention the other available option.
• Adds the shared pre-reqs for running Flows
• Duplicates the instructions for setting up automated reviews and custom instructions from the classic code review page to avoid user confusion.

Background info

There are two code review features now:

• GitLab Duo Code Review (for Duo Enterprise only)
• Code Review Flow (for Duo Core and Duo Pro)

You can learn more about the decision and plan in https://gitlab.com/gitlab-org/gitlab/-/work_items/579921+

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.