Fix database storage layout depth in architecture docs

The tree was missing the {session_id} level between sessions/ and its
children (runs/, solutions/, photoscans/). Also added per-object ID
subdirectories ({run_id}/, {solution_id}/, {photoscan_id}/) to match
the actual paths used in database.py.

Refs #129, #338

Author: Brandon Kirkland

docs/architecture.rst

@@ -438,10 +438,14 @@ Storage Layout
    │   └── {subject_id}/
    │       ├── volumes/
    │       └── sessions/
-   │           ├── runs/
-   │           ├── solutions/
-   │           ├── photoscans/
-   │           └── photocollections/
+   │           └── {session_id}/
+   │               ├── runs/
+   │               │   └── {run_id}/
+   │               ├── solutions/
+   │               │   └── {solution_id}/
+   │               ├── photoscans/
+   │               │   └── {photoscan_id}/
+   │               └── photocollections/
    ├── protocols/
    ├── transducers/
    ├── systems/