Document file entities in the data model.

docs/model/type/file.md

@@ -0,0 +1,5 @@
+# Files
+
+Files can be uploaded to farmOS and associated with
+[Assets](/model/type/asset), [Logs](/model/type/log), and
+[Plans](/model/type/plan).

mkdocs.yml

@@ -7,6 +7,7 @@ nav:
       - Logs: model/type/log.md
       - Quantities: model/type/quantity.md
       - Data streams: model/type/data_stream.md
+      - Files: model/type/file.md
       - Terms: model/type/term.md
       - Plans: model/type/plan.md
       - Users: model/type/user.md