Add NVIDIA RAG Blueprint to skills catalog

[kumar-punit]

Add the RAG Blueprint skill to the Available Skills table and Getting Help & Contributing section. The skill enables AI agents to deploy, configure, troubleshoot, and manage the NVIDIA RAG pipeline via Docker Compose or Helm.

[mosheabr]

LGTM @kumar-punit — verified against the source repo:

• Skill count (1) is correct — single rag-blueprint skill directory
• Skill structure is solid — SKILL.md + references/ with deploy, configure, troubleshoot, and shutdown guides (~30KB of reference material)
• Hosted under NVIDIA-AI-Blueprints org — consistent with existing catalog entries
• Contributing/Issues/Discussions/Security links all correct

One heads up: PR #19 is about to land which moves the catalog to a config-driven products.yml. After that merges you'll want to add a products.yml entry for RAG Blueprint so the sync workflow picks it up automatically. But that can be a follow-up.

Approving — merging now.