docs(Pajamas): Add pattern page for Duo slash commands

What does this MR do?

This MR creates a new folder under Patterns called GitLab Duo to house Duo and AI-specific pattern guidance. This MR also contributes a page about slash commands in Duo Chat. This MR is the first step towards establishing an AI UX pattern library. The intention to document UI patterns, guidelines, and best practices for AI features.

Problem this guidance addresses

Full context in Figma

The primary goal behind this effort is making sure slash commands are added to Duo Chat in a scalable, holistic way. The concern I’m trying to mitigate is individual teams will add slash commands of varying levels of abstraction and we will end up with a long list of commands that are hard to understand.

What does this MR do?

This MR introduces guidance around:

• What should / should not be a slash command in Duo Chat
• Naming convention and syntax of slash commands in Duo Chat
• Behavior and layout of slash command menu in Duo Chat

Does this MR meet the acceptance criteria?

• The MR title and commit messages meet the Pajamas commit conventions.
• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• If creating a new component page from scratch, it follows the page template structure.
• Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.
• Maintainer review follows the Pajamas UX maintainer review checklist.

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