[FIX] mention phenotype dir in folder structure (#1644)

* mention phenotype dir in folder structure

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/schema/objects/files.yaml

@@ -102,6 +102,15 @@ derivatives:
     Derivative data (for example preprocessed files).
     See the [relevant section](SPEC_ROOT/common-principles.md#source-vs-raw-vs-derived-data)
     for more information.
+phenotype:
+  display_name: Phenotype
+  file_type: directory
+  description: |
+    A directory where to store participant level measurements (for
+    example responses from multiple questionnaires) split into
+    individual files separate from `participants.tsv`.
+    See the [relevant section](SPEC_ROOT/modality-agnostic-files.md#phenotypic-and-assessment-data)
+    for more information.
 sourcedata:
   display_name: Source data
   file_type: directory