Add custom instructions for Duo code review

Custom instructions for Duo can improve quality of AI code reviews in the project.

Let's try adding a config file to this project.

Some things we might want to specify:

• Prefer use of prefixed Tailwind classes over adding more CSS properties
• Flag English strings in Hugo templates and Vue templates
• Use helper methods like docsBaseURL() in JS when referencing absolute paths
• Shortcodes require error handling (and we have a shared error handler we can use for this)
• GitLab-specific guidelines (see https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/duo/mr-review-instructions.yaml for ideas)
• Anything else? It might help to look at merged MRs that had a lot of comments/back-and-forth to see if there are any other recurring things that always come up in review (or suggestions Duo has made that are always off)

Docs: https://docs.gitlab.com/user/project/merge_requests/duo_in_merge_requests/#customize-instructions-for-gitlab-duo-code-review