message: Document Substitutions In The Descriptor Format
diff: @@ -93,6 +93,41 @@ significance and have reserved purposes and distinct shapes.
  See below for details about these fields.
  +## Substitutions In The Descriptor Format
  +
  +The descriptor format supports substitutions using `$text`, `$json`, and
  +`$yaml`.
  +
  +Placeholders like `$json: https://example.com/entity.json` are substituded by
  +the content of the referenced file. Files can be referenced from any configured
  +integration similar to locations by passing an absolute URL. It's also possible
  +to reference relative files like `./referenced.yaml` from the same location.
  +There are three different types of placeholders:
  +
  +- `$text`: Interprets the content of the referenced file as plain text and embed
  + it as a string.
  +- `$json`: Interprets the content of the referenced file as JSON and embed the
  + parsed structure.
  +- `$yaml`: Interprets the content of the referenced file as YAML and embed the
  + parsed structure.
  +
  +For example, this can be used to load the definition of an API entity from a web
  +server and embed it into the entity:
  +
  +```yaml
  +apiVersion: backstage.io/v1alpha1
  +kind: API
  +metadata:
  + name: petstore
  + description: The Petstore API
  +spec:
  + type: openapi
  + lifecycle: production
  + owner: petstore@example.com
  + definition:
  + $text: https://petstore.swagger.io/v2/swagger.json
  +```
  +
  ## Common to All Kinds: The Envelope
  The root envelope object has the following structure.
  