[codex] Rework NeMo OSS docs orientation

[lbliii]

Summary

• Restructure the NeMo OSS Fern hub around task-first and runtime-first orientation
• Convert Concepts into a section with focused concept pages and add Resources pages for glossary and external learning
• Update the NVIDIA-NeMo profile README as a concise public landing page with links into the hub
• Clean public-facing copy to remove maintainer/design-process language

Validation

• fern check
• git diff --check