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>

mkdocs.yml

@@ -0,0 +1,41 @@
+site_name: register-referentie
+site_description: Reference application for Respellion's Common Ground architecture pattern
+docs_dir: docs
+
+theme:
+  name: material
+  features:
+    - navigation.sections
+    - navigation.top
+    - content.code.copy
+  palette:
+    - scheme: default
+      toggle:
+        icon: material/brightness-7
+        name: Switch to dark mode
+    - scheme: slate
+      toggle:
+        icon: material/brightness-4
+        name: Switch to light mode
+
+nav:
+  - Home: index.md
+  - Product Requirements: PRD.md
+  - Architecture:
+      - "ADR-0001: Loose coupling": architecture/adr-0001-loose-coupling.md
+  - Working in Gitea: gitea-workflow.md
+  - Runbooks:
+      - CI: runbooks/ci.md
+
+markdown_extensions:
+  - admonition
+  - toc:
+      permalink: true
+  - pymdownx.superfences
+
+# Many docs referenced by PRD.md land in later slices; don't fail the build on them.
+validation:
+  nav:
+    omitted_files: warn
+  links:
+    not_found: warn