Add ARD (Agentic Resource Discovery) catalog

[elvinagam]

Adds an Agentic Resource Discovery (ARD) catalog so external agents/tools can discover NVIDIA's skills programmatically, instead of only through the skills/ directory listing.

• docs/.well-known/ai-catalog.json — ARD-compliant catalog with one entry per skill in skills/ (identifier, display name, description, and representative search queries for each)
• .github/workflows/publish-catalog.yml — publishes docs/ (including the catalog) to GitHub Pages on push to main
• .github/workflows/validate-ai-catalog.yml — runs the integration tests on PRs/pushes that touch the catalog, skills/**/SKILL.md, or the test file itself
• tests/integration/test_catalog.py — asserts the catalog is valid JSON, every skill in skills/ has a matching entry (and vice versa, no stale entries), and every entry has the required ARD fields (identifier, type, url, description, representativeQueries)
• CONTRIBUTING.md — documents the checklist for adding a catalog entry when contributing a new skill
• CHANGELOG.md — notes the addition under [Unreleased]

Test plan

• pytest tests/integration/test_catalog.py passes locally (5/5)
• validate-ai-catalog.yml workflow passes in CI
• After merge, confirm publish-catalog.yml deploys and docs/.well-known/ai-catalog.json is reachable at the published GitHub Pages URL

Onboarding type

• New product onboarding (new components.d/<slug>.yml file)
• Other (catalog change, README fix, infrastructure, etc.)

For new product onboarding — author affirmations

By submitting this PR, I confirm on behalf of my team:

• Skills cleared for open source release per NVIDIA's internal IP review process (six-question check, all answers affirmative)
• License selected: Apache 2.0 / CC-BY 4.0 / Dual (Apache 2.0 + CC-BY 4.0). Specify: _____
• No new license or new third-party component introduced beyond what the source repo already carries
• Source repo is public and under an NVIDIA-owned GitHub org
• .agents/skills/ or skills/ path used for new entries (or existing path retained for legacy entries per components.d/<slug>.yml)

NVIDIA contributors: see the internal onboarding guide for the IP review process details and license selection.

Reviewer checklist (OSS Skills PIC)

• Author confirmations above are checked
• components.d/<slug>.yml entry valid (required fields, unique catalog_dir, path exists in source repo, filename slug matches name)
• SKILL.md frontmatter spec-compliant (at least one sampled)
• No new license or third-party dependency requiring OSRB filing

All PRs

• All commits signed off with DCO (git commit -s).
  If you forgot, run git rebase --signoff origin/main && git push --force-with-lease to retroactively sign all commits in your branch.

Other context (for non-onboarding PRs)