docs(specs): add IO dependencies doc

docs/source/specifications/io_dependencies.org

+#+title: Input/Output Stage Dependencies
+
+Overview of what fields are required for each consecutive step to run, and
+
+- Registration
+- Tiler
+  - Requires:
+    - None
+  # - Optionally:
+  - Produces:
+    - /trap_info
+- Tiler
+  - Requires:
+    - None
+  - Produces:
+    - /trap_info