fix(docs): add mkdocs.yml at repo root and minimal page stubs

[adris-misra]

Phase

Phase 9 follow-up: MkDocs site config

Summary

Resolves the post-merge Build MkDocs Site failure on main (commit
9a740da). The docs-publish workflow was added in Phase 1 but the
mkdocs.yml config it depends on was never created — Phase 1 spec §3.2
listed it under docs/, but the standard MkDocs convention (and what the
workflow expects) is at the repo root.

What changed

• mkdocs.yml at repo root — Material theme, full nav, pymdownx extensions
• docs/architecture/overview.md — 10-agent pipeline diagram, design
  principles, agent roles table
• docs/architecture/uns_context_broker.md — zone enforcement, topic
  format reference, API example
• docs/standards/standards_alignment.md — full standards matrix
  (ISA-95, MQTT Sparkplug B, OPC UA, NIST SP 800-82, CMMC L2, NIST AI RMF,
  ISO 42001, ISO 14224)
• docs/governance/ai_rmf_profile.md — GOVERN/MAP/MEASURE/MANAGE
  function mapping with code examples
• docs/case_studies/smm_starter.md — 5-minute local Ollama quickstart
  for SMMs (NIST MEP-audience targeted)
• docs/case_studies/chips_supplier.md — Bedrock/GovCloud deployment
  for CHIPS Act Tier-2/Tier-3 suppliers
• pyproject.toml — added [docs] extras group with mkdocs-material>=9.5
  and pymdown-extensions>=10.0
• docs/index.md and docs/governance/cmmc_l2_mapping.md left untouched
  (already existed with substantive content)

Acceptance criteria covered

• mkdocs build --strict passes with zero warnings
• All nav entries reference existing files
• Standard MkDocs convention (config at repo root, docs_dir: docs)
• No changes to existing docs with substantive content
• No changes to the docs-publish workflow itself

How to verify

pip install -e ".[docs]"
mkdocs build --strict
mkdocs serve  # optional, opens local preview at :8000

Why this matters for the project

GitHub Pages will auto-deploy on merge, producing a citable docs URL at
https://adris-misra.github.io/multi-agentic-framework/ — the docs site
itself becomes a public, dated, citable artifact.

Out of scope

• Substantive content authoring for docs/index.md and cmmc_l2_mapping.md
  beyond what's already there
• Whitepaper polish (separate follow-up)