docs: initial docs/ infrastructure setup

[miyoungc]

Description

Set up the foundational documentation infrastructure for NemoClaw using Sphinx/MyST with the NVIDIA theme.

Scope

• docs/conf.py — Sphinx configuration with nvidia-sphinx-theme, MyST parser, sphinx-design, mermaid, copybutton
• docs/index.md — Landing page with toctree structure
• Section directories with placeholder index pages: about/, get-started/, feature1/, feature2/, observability/, references/, troubleshooting/, resources/
• pyproject.toml — docs dependency group (uv/PEP 735 style)
• Makefile — docs, docs-strict, docs-serve, docs-clean targets using uv run
• build/docs.toml — Equivalent mise tasks
• .gitignore — _build/ added
• GitHub Actions workflows:
  • docs-build.yaml — Build on PRs to main and pushes, upload artifact, publish to gh-pages
  • docs-preview-pr.yaml — PR preview comments via nvidia-merlin shared workflow
  • docs-remove-stale-reviews.yaml — Weekly stale preview cleanup

Definition of Done

• make docs-strict builds with zero warnings
• make docs-serve starts a live-reload server on port 8001
• CI workflows trigger correctly on docs-related PRs

[miyoungc]

NVIDIA/NemoClaw#94