[ENH] Formalize presence of optional docs/ folder

src/common-principles.md

@@ -392,6 +392,7 @@ Derivatives can be stored/distributed in two ways:
     that were used to generate the derivatives.
     Likewise, any code used to generate the derivatives from the source data
     MAY be included in the `code/` subdirectory.
+    Extra documentation (and relevant images) MAY be included in the `docs/` subdirectory.
     Logs from running the code or other commands MAY be stored under `logs/` subdirectory.
 
     Example of a derivative dataset including the raw dataset as source:

src/schema/rules/directories.yaml

@@ -34,6 +34,10 @@ raw:
     name: derivatives
     level: optional
     opaque: true
+  docs:
+    name: docs
+    level: optional
+    opaque: true
   logs:
     name: logs
     level: optional

src/schema/rules/files/common/core.yaml

@@ -36,6 +36,9 @@ code:
 derivatives:
   level: optional
   path: derivatives
+docs:
+  level: optional
+  path: docs
 logs:
   level: optional
   path: logs