Vale: test for "Duo" without preceding "GitLab"

What does this MR do?

Adds this line to the error-level doc/.vale/gitlab/Substitutions.yml to test for instances of Duo without a preceding GitLab:

'(?<!GitLab) Duo': GitLab Duo

With these changes, this rule adds no new errors to the docs:

• Fixed a false negative in doc/development/ai_features/duo_chat.md
• Disabled the test around the appropriate parts of doc/user/profile/account/two_factor_authentication.md because we support Duo as an OTP provider
• Unfixed in this MR because we don't test the page: doc/api/graphql/reference/index.md (requires long pipeline to fix; I've created Fix API definitions / docs to use "GitLab Duo" (!131709 - merged) for it)

I used https://pkg.go.dev/github.com/s-kozlov/goback/regexp/syntax because I can never remember the lookahead/lookbehind syntax on the first try.

Related issues

• Related to Adding AI terms (!131440 - merged) and follows Suzanne's guidance in !131440 (comment 1553814970).
• Related to https://gitlab.com/gitlab-com/marketing/brand-product-marketing/product-marketing/-/issues/7158+
• Related to https://gitlab.com/gitlab-org/gitlab/-/issues/424793+
• Related to Fix API definitions / docs to use "GitLab Duo" (!131709 - merged) which fixes API definitions deliberately skipped here to preserve a short pipeline

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• 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.