docs(architecture): import the FDS architecture decisions from the lab repo (closes #159) (#160)

## What & why

Brings the engineer-facing FDS documentation next to the code it describes. Imported from `projects/open-register-fd/` in `Respellion/innovation-lab` and translated to Dutch: **six ADRs**, the ADR index and template, the **L3 component view**, and the **slice-1 proposal**.

The architecture blueprint, the FDS gap analysis and the two privacy views stay in the lab repo — the OKRs cite them and they feed tender responses. Each side names the split in a "Wat ligt waar" table, so nothing is documented twice.

Closes #159

### Why `docs/architecture/fds/` and not `docs/architecture/`

This repo's own ADR series now runs `adr-0001-loose-coupling` … `adr-0010-bff-oidc`. The imported set is numbered 0001–0006, so a flat import would collide across the whole imported range. The subfolder preserves the imported numbering, and with it roughly thirty `ADR-000N` cross-references inside the imported text that would otherwise all need rewriting.

In the MkDocs sidebar the imported six appear as **FDS ADR-000N** so they are not confused with this repo's series. `docs/architecture/fds/README.md` explains the two series.

### Mermaid support was missing

`pymdownx.superfences` had no `custom_fences`, so the imported diagrams would have published to Gitea Pages as raw code blocks. This PR adds the mermaid custom fence, the nav group, and one link under *Where to go* in the docs index.

## Definition of Done

- [x] Linked Gitea issue (above).
- [ ] Failing test committed before the implementation. — n/a, documentation only.
- [ ] Implementation makes the test pass. — n/a, documentation only.
- [x] Conventional Commits referencing the issue (`refs #159`).
- [x] Rebased on current `main`; no conflicts.
- [ ] CI green — n/a for content; the docs verification is below.
- [ ] `docker compose up` reaches green health checks. — n/a, no runtime change.
- [x] Docs updated if behaviour, contracts, or operations changed.
- [x] ADR added in `docs/architecture/` if a non-obvious decision was made. — six imported, plus the numbering decision recorded in the folder README.
- [ ] Demo note in `docs/demo-script.md`. — n/a, nothing user-visible.

## Verification run

- `mkdocs build` — clean. No missing-nav warning for any `architecture/fds/` entry. The two remaining warnings are pre-existing on `main` and untouched here: the set of pages absent from `nav`, and a broken link in `runbooks/ci.md` to `services/acl/stryker-config.json`.
- Mermaid renders as a diagram, not a code block: `site/architecture/fds/c4-component-view/index.html` contains `class="mermaid"`.
- All relative markdown links in the repo resolve.

## Notes for reviewers

- **Language.** The imported documents are Dutch; this repo's own documents remain English. Deliberate, not an oversight — the lab repo standardised on Dutch and these pages moved with it. Translating the rest is a separate decision.
- **Ownership.** This repo sits in the `eho/` namespace while it now holds the canonical FDS architecture decisions that tender answers point at. Worth deciding whether it should move to `Respellion/`.
- **Scope drift, not fixed here.** The imported text is faithful to its source, so the slice-1 proposal and the ADRs assume NHR/KVK for slice 1, while the lab-side blueprint still uses BAG as its example register. The lab-side documents carry a banner about this; Blueprint v2 (slice 5) is where the diagrams get corrected.
- **Companion PR:** `Respellion/innovation-lab` #34 holds the lab-side half of this split.Reviewed-on: #160

mkdocs.yml

@@ -32,6 +32,17 @@ nav:
       - "ADR-0008: Read projection store": architecture/adr-0008-read-projection-store.md
       - "ADR-0009: External-task job worker": architecture/adr-0009-external-task-job-worker.md
       - "ADR-0010: BFF OIDC validation": architecture/adr-0010-bff-oidc.md
+  - FDS-architectuur:
+      - Overzicht: architecture/fds/README.md
+      - Componentview (L3): architecture/fds/c4-component-view.md
+      - "Slice 1: walking skeleton": architecture/fds/slice-1-proposal.md
+      - "FDS ADR-0001: ACL op elke registergrens": architecture/fds/adr/0001-acl-at-every-register-boundary.md
+      - "FDS ADR-0002: FSC voor connectiviteit": architecture/fds/adr/0002-fsc-for-connectivity.md
+      - "FDS ADR-0003: PBAC via OPA": architecture/fds/adr/0003-pbac-via-opa.md
+      - "FDS ADR-0004: Begrensde cache": architecture/fds/adr/0004-bounded-cache.md
+      - "FDS ADR-0005: Verwerkingenlog via events": architecture/fds/adr/0005-ldv-verwerkingenlog.md
+      - "FDS ADR-0006: Modulegrens en hergebruik": architecture/fds/adr/0006-module-boundary-and-reuse.md
+      - "FDS ADR-template": architecture/fds/adr/template.md
   - Working in Gitea: gitea-workflow.md
   - Frontend decisions: frontend-decisions.md
   - Demo script: demo-script.md
@@ -42,7 +53,11 @@ markdown_extensions:
   - admonition
   - toc:
       permalink: true
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_code_format
 
 # Many docs referenced by PRD.md land in later slices; don't fail the build on them.
 validation: