feat(AI): create AI CTA page

What does this MR do?

This MR adds a new page for AI CTAs under the GitLab Duo section in patterns. The goal of this MR is to create a central place of AI CTA guidelines, with the aim of making AI CTAs more consistent.

I've tried to make this MR as small as possible. I am planning to add additional guidance to this page in several smaller MRs. Here are the planned follow-ups: #1881 (closed)

Related issues:

• [AI pattern] Guidelines for AI calls to action ... (gitlab-org/gitlab#469062 - closed)
• Pajamas documentation > follow-up to AI CTA gui... (#1881 - closed)

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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