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/index.md

@@ -0,0 +1,23 @@
+# 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.