message: docs: add section to help decide where to place code
diff: @@ -256,6 +256,18 @@ The Backstage CLI is in a category of its own and is depended on by virtually
  all other packages. It's not a library in itself though, and must always be a
  development dependency only.
  +### Deciding where you place your code
  +
  +It can sometimes be difficult to decide where to place your plugin code. For example
  +should it go directly in the `-backend` plugin package or in the `-node` package?
  +As a rule of thumb you should try to keep the exposure of your code as low
  +as possible. If it doesn't need to be public API, it's best to avoid. If you don't
  +need it to be used by other plugins, then keep it directly in the plugin packages.
  +
  +Below is a chart to help you decide where to place your code.
  +
  +![Package decision](../assets/architecture-overview/package-decision.drawio.svg)
  +
  ## Databases
  As we have seen, both the `lighthouse-audit-service` and `catalog-backend`
  