Expand NVIDIA skills docs overview

[swaminator]

Summary

• Reframe the docs landing page around NVIDIA-verified skills as capability governance for portable agent skills.
• Add a clearer overview of what NVIDIA skills help agents do, with examples of product areas and workflow groups rather than trying to maintain an exhaustive available-skills list in docs.
• Link to skills.sh and NVIDIA Build as the source of truth for the current catalog.
• Add cross-agent delivery messaging for Vercel's skills CLI, including a link to the supported agents list, plus marketplace and partner hub discovery paths.

Intent

We wanted the docs to give users an overview of what skills are available for NVIDIA and where to discover/install them, with links to the relevant catalog and delivery surfaces, instead of only describing what verified skills are.

Validation

• git diff --check