Expand docs on configuration file structure

docs/data.md

@@ -1,3 +1,28 @@
 User data is initially configured to $HOME/.config/tori. At this stage, this allows concentrating all necessary data in a single place, however it is not ideal to have non-configuration file there.
 
 When making these configurable, it would be interesting to move the default backup location to $HOME/.local.
+
+The user data is organized in two main directories: `base` and `bkp`.
+
+- `base`: contains the files that will be matched against the current system's files
+- `bkp`: contains the original files prior to any intervention by `tori`. This directory is further split into two directories:
+  - `canonical`: contains the very first version found for this file prior to any intervention by `tori`. If `tori` is running for the first time on a brand new system, this directory should contain the original files shipped by the operating system.
+  - `ephemeral`: contains date-stamped versions of files prior to each occasion in which `tori` had to modify, delete or move them. This directory is not essential to the functioning of `tori`. It is provided as a safety net for the user and may be deleted at any time. It will be automatically recreated.
+
+Both of these directories mimic the file structure found from the root of the file system. For example, this is how one such structure could look like:
+
+```
+/home/user/.config/tori
+├── base
+│   └── home
+│       └── user
+            ├── .profile
+│           └── .shrc
+└── bkp
+    └── canonical
+        └── home
+            └── user
+                ├── .profile
+                └── .shrc
+```
+