docs: Phase 1 content IA — frontmatter, Start here, CLI quickstart

[kheiss-uwzoo]

Summary

• Add YAML frontmatter (content_type, audience, journey_stage, prerequisites) to four anchor pages: overview, deployment options, document ingestion workflow, and multimodal extraction.
• Promote About this section to Start here with a Diataxis-typed onboarding table and durations.
• Add Documentation on GitHub (external-documentation-map.md) listing canonical NeMo-Retriever GitHub surfaces for Helm, CLI, Docker, and graph customization.
• Replace the snippet-only CLI quickstart with a self-contained in-repo tutorial (install, ingest, query, next steps).

Internal IA audit and Fern migration plan are not included in this PR.

Test plan

• mkdocs build -f mkdocs.yml --strict from docs/
• check-nrl-doc-leakage.ps1 pass
• git diff upstream/main..HEAD --name-only shows customer docs only (8 files)
• Manual: Start here to workflow to CLI path readable in built site