docs(architecture): import the FDS architecture decisions (refs #159)

Bring the engineer-facing FDS documentation next to the code it
describes: ADR-0001 to ADR-0006, the ADR index and template, the L3
component view, and the slice-1 proposal. All translated to Dutch.
Source: projects/open-register-fd/ in Respellion/innovation-lab.

Land the set in docs/architecture/fds/ rather than docs/architecture/.
This repo already owns adr-0001-loose-coupling to adr-0004-bdd-framework,
so a flat import collides on every number. The subfolder keeps the
imported numbering, and with it about thirty ADR-000N cross-references
in the imported text.

Add the mermaid custom fence to pymdownx.superfences. Without it the
imported diagrams publish as raw code blocks, because the site has no
mermaid support today. Add the nav group and one link from the docs
index.

The blueprint, the FDS gap analysis and the privacy views stay in the lab
repo; the OKRs cite them and they feed tender responses.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/index.md

@@ -9,6 +9,9 @@ should teach.
 - **[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.
+- **[FDS architecture](architecture/fds/README.md)** — participating in the Federatief
+  Datastelsel as an afnemer: ADR-0001…0006, the L3 component view, the slice-1 proposal.
+  In Dutch; the strategic framing lives in `Respellion/innovation-lab`.
 - **[Working in Gitea](gitea-workflow.md)** — issues, milestones, branches, PRs.
 - **[CI runbook](runbooks/ci.md)** — the pipeline and the `make ci` local gate.