Phase 5 — Documentation parity & FlowFuse-authored docs

[n-lark]

Phase 5 — Documentation parity & FlowFuse-authored docs

Depends on Phase 4 — overlay implementation should land in Nuxt, not Eleventy.

Goal: Detail pages must be authoritative documentation for FlowFuse's certified and proprietary nodes, not just a wrapper around an upstream README. Three cases:

1. Forked nodes (@flowfuse-certified/<name>) — we've shipped fixes/changes ahead of upstream. Users need our docs, with the changes called out.
2. Proprietary nodes (e.g. OPC UA) — no public upstream. This page IS the documentation.
3. Certified nodes with thin upstream docs — we want to add supplemental content without forking.

Tasks:

• Audit each certified node (README, example flows from integrations.js:108-187, inline help). Classify each as upstream-sufficient, needs-overlay, or FlowFuse-authored.
• Build the overlay pattern — per-node markdown in this repo (e.g. src/integrations/_overrides/<node-id>.md) that replaces or augments the rendered README.
• Add the divergence callout — visual flag on forked-node pages: "FlowFuse-maintained documentation. Differs from upstream — see what's changed."
• Use OPC UA as the proof case. First FlowFuse-authored page; validate the pattern before scaling.
• Establish a sync process for keeping overlays current. Owner TBD.

Acceptance: OPC UA has a complete FlowFuse-authored page. The overlay/callout pattern works for any certified node. Forked-node pages clearly distinguish FF-authored content from upstream.