Document a new processing

A new ProcessingType in src/types.ts stops the docs build with “Missing docs page for processing type” until you finish these steps.

1. Add the type to PROCESSING_GROUP in site/src/data/processings.ts.
2. Add its machine-file row to site/src/data/exportCoverage.ts, from docs/format-matrix.md.
3. Copy a page in site/src/content/docs/processings/, rename it, and set processingType, slug and sidebar.order.
4. Write only what the code or the internal docs confirm. Field notes come from the app’s paramNoteKeys; add others with notes on ParamTable. If a field’s meaning is unclear, set status: draft and say why in todo.
5. Run npm run docs:check.

The figure, the field table and the default values come from the app code, so they need no work.