Document agent skills in Agentic Onboarding Setup

[brett0000FF]

What does this PR do? What is the motivation?

Adds an Agent skills section to Agentic Onboarding Setup. Explains the orchestrator and installation first, then lists entry-point skills by exact name with one-line descriptions. Links to supporting workflows and the Agent Observability and Software Delivery collections keep the setup page concise.

Merge readiness

• Ready for merge

AI assistance

Codex researched the public skill definitions, drafted the documentation, and ran validation checks.

Additional notes

• Vale: no alerts on changed lines; six existing alerts on untouched content.
• Verified all 17 catalog names against their skill definitions, covering all 15 top-level skills plus Datadog Apps and resource ownership. Checked source links, collection links, internal documentation links, and YAML front matter.
• Ran the installer in list-only mode against the reviewed source and confirmed the documented name-selection workflow.
• Rendered the new section with Hugo and checked the catalog, concise orchestrator description, anchors, and code blocks.
• Repository pre-commit checks and git diff --check passed. Commits signed and signatures verified.
• Full-site local build could not complete because the checkout lacks generated integration image data and the markdoc-assets.html partial. The PR preview still needs review.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/codex/document-agent-skills/agentic_onboarding/setup