docs: MkDocs scaffold, ADR-0001, README quickstart (refs #32)

Add mkdocs.yml (Material theme + nav), docs/index.md, and the first ADR
(docs/architecture/adr-0001-loose-coupling.md, Nygard template) recording
the loose-coupling stance from CLAUDE.md §8. Rewrite the README quickstart
to be accurate and reach a green local environment in under 10 minutes
(correct clone URL, .NET 10 + container-engine prereqs, `make ci` and the
compose smoke), and document building the docs site. Ignore .venv/ and site/.

Verified: `mkdocs build` succeeds; `make lint/build/unit` green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/architecture/adr-0001-loose-coupling.md

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+# ADR-0001: Loose coupling to upstream Common Ground modules
+
+- **Status:** Accepted
+- **Date:** 2026-06-03
+- **Deciders:** Respellion engineering
+- **Template note:** This is the first ADR and doubles as the worked example of
+  the Nygard template. Copy its shape for new ADRs (`adr-NNNN-title.md`).
+
+## Context
+
+This reference application orchestrates several upstream Common Ground modules —
+OpenZaak (ZGW APIs), Open Notificaties (NRC), Objecten/Objecttypen, Flowable,
+Keycloak. Each is an independently developed, independently deployed peer. The
+temptation in a demo is to reach straight into a peer's database or couple to its
+internal schema to move faster. That coupling is exactly what makes Common Ground
+landscapes brittle and un-upgradeable in practice.
+
+We need a stance, recorded up front, on how our services may talk to these peers.
+
+## Decision
+
+**We integrate with upstream modules only through their documented public APIs, and
+we isolate that integration behind explicit anti-corruption boundaries.**
+
+Concretely (mirrors CLAUDE.md §8):
+
+1. The **ACL** is the only code that talks to ZGW APIs; no other service constructs
+   ZGW URLs.
+2. The **Workflow Client** is the only code that talks to Flowable; BPMN models hold
+   no OpenZaak knowledge.
+3. **Portals talk only to the BFF** — never directly to a backend or a peer module.
+4. **No direct database access across services or to any peer.** Each service owns
+   its schema; the Read Projection is a rebuildable derived artefact.
+5. **Idempotency at every event boundary** (the Event Subscriber tolerates duplicate
+   and out-of-order NRC events).
+
+Bending any of these is an ADR-worthy moment (CLAUDE.md §14): stop and open an
+`adr-proposal` issue first.
+
+## Consequences
+
+**Positive**
+
+- Upstream modules can be upgraded or swapped behind their APIs without rippling
+  through our services.
+- Coupling is visible and minimal — anti-corruption code lives in one named place.
+- The architecture teaches the Common Ground pattern by enforcing it.
+
+**Negative / costs**
+
+- More indirection: a translation layer (ACL, Workflow Client) instead of direct
+  calls. Accepted — it's the point.
+- Eventual consistency across aggregates must be designed for, not assumed away.
+
+**Follow-up**
+
+- Each integration slice that touches a boundary references this ADR; new boundary
+  decisions get their own ADR.

docs/index.md

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+# register-referentie
+
+A reference application demonstrating Respellion's Common Ground architecture
+pattern. Quality and architectural clarity over feature throughput — every commit
+should teach.
+
+## Where to go
+
+- **[Product Requirements](PRD.md)** — what we're building and why.
+- **[ADR-0001: Loose coupling](architecture/adr-0001-loose-coupling.md)** — the
+  non-negotiable integration stance; the template for future ADRs.
+- **[Working in Gitea](gitea-workflow.md)** — issues, milestones, branches, PRs.
+- **[CI runbook](runbooks/ci.md)** — the pipeline and the `make ci` local gate.
+
+## Quickstart
+
+See the repository `README.md`. In short: clone, then either run the checks with
+`make ci`, or bring the BFF up with
+`docker compose -f infra/docker-compose.yml up -d --build --wait` and
+`curl http://localhost:8080/health`.
+
+> This site is built with MkDocs Material (`mkdocs build`). It grows with the
+> backlog; sections appear as their slices land.