docs: add agentic docs workflow guidance (AGENTS.md)

[kheiss-uwzoo]

Summary

• Add root AGENTS.md so coding agents detect user-visible code changes and route documentation work in parallel (or inline when subagents are unavailable).
• Add docs/AGENTS.md with a compact NVIDIA Writing Style Guide and NRL-specific doc patterns for MkDocs extraction docs.
• Add docs/DORI_SETUP.md for explicit DORI install/config only (no auto-install; no MCP/host changes in this PR).

Notes

• Implements Eric’s docs-agent-setup pilot surfaces for NeMo-Retriever ahead of 26.07 RC QA.
• Does not add CI/hooks, product skills under .agents/, or CLAUDE.md changes. Existing tracked .agents file (.claude) is left unchanged.

Test plan

• Review the three new files for accuracy against NRL docs layout (docs/docs/extraction/, MkDocs).
• Confirm PR diff is only AGENTS.md, docs/AGENTS.md, docs/DORI_SETUP.md.
• Optional: in a Cursor session on this branch, confirm agents load root/docs AGENTS guidance.
• Do not run DORI install or host MCP setup as part of verifying this PR.