Add certified nodes process to handbook

src/handbook/development/certified-nodes.md

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+---
+navTitle: Certified Nodes
+---
+
+FlowFuse maintains a catalogue of nodes from the community that we put through regular checks
+to ensure they are of high quality and standards for use by our customers.
+
+The catalogue is maintained in the [certified-nr-nodes](https://github.com/FlowFuse/certified-nr-nodes)(🔒) repository.
+
+A daily GH action runs the audit process across all nodes and regenerates the `catalogue.json` file served to Node-RED
+instances within FlowFuse.
+
+## Proposing a new node to be included
+
+1. [Raise an issue](https://github.com/FlowFuse/certified-nr-nodes/issues/new)(🔒) with details of the node to be added.
+2. Assign the issue to the CTO (or in his absence, another member of the Engineering Team).
+
+## Adding a new node to the list
+
+Full details on the technical process are provided in the [certified-nr-nodes](https://github.com/FlowFuse/certified-nr-nodes)(🔒) readme.
+
+1. Perform due diligence on the module; how popular is it, is it well maintained, is there a backlog of open issues etc.
+2. Add the module to the list held in `modules.json`
+3. Run the audit locally to check the current state of the module. Review the results and assess whether we can accept it into the catalogue in its current state.
+4. Raise a PR with the updated module list and audit output for the module.
+5. Review the PR with the CTO/Engineering Team
+
+