docs: document managed generation on the Artifacts page - #1142

What

Extend the existing Agent Interface Artifacts page with a Generate and edit managed slides and reports section. No new documentation page or navigation entry is introduced.

Changes

• Explain when to use managed artifacts inside a Responses conversation versus the standalone Artifact Chat Completions endpoint.
• Move the original PR's four code examples into /docs/agent/core-concepts/artifacts#generate-and-edit-managed-slides-and-reports, covering authentication, JSON-string metadata, generation, managed viewers, streaming, and edit requests.
• Clarify the server-only API key boundary, server-to-browser streaming handoff, and application-owned persistence and authorization for standalone artifacts.
• Keep the existing custom-artifact guidance separate, with links to the dedicated custom renderer guide.
• Link directly to the new section from Chat Completions.
• Redirect /docs/openui-cloud/api/artifacts to that section before the broader Cloud redirect.

Test Plan

• Compiled and server-rendered both changed MDX pages using the installed MDX compiler and Fumadocs heading/GFM plugins.
• Checked all 11 internal links in those rendered pages, including the new section anchor.
• Confirmed all four original code examples are preserved unchanged.
• Tested the legacy URL with Next.js's route-config test helper: HTTP 308 to the Artifacts section; the broader Cloud catch-all remains unchanged.
• Checked the managed conversation wiring against the default CLI template and standalone viewer props against @openuidev/thesys@0.3.2 declarations.
• Prettier and git diff --check passed.

No full documentation-site build or live artifact API calls were run for this revision.

Checklist

• I linked a related issue, if applicable (no related issue)
• I updated docs/README when needed
• I considered backwards compatibility