docs(configuration): stub out top-level config

Author: tony

docs/configuration/top-level.md

@@ -0,0 +1,42 @@
+(top-level)=
+(top-level-config)=
+
+# Top-level configuration
+
+## `session_name`
+
+Used for:
+
+- tmux session name
+- checking for existing sessions
+
+Notes:
+
+- May differ from the config filename
+
+  e.g. _apple.yaml_:
+
+  ```yaml
+  session_name: banana
+  windows:
+    - panes:
+        -
+  ```
+
+  Load detached:
+
+  ```console
+  $ tmuxp load ./apple.yaml -d
+  ```
+
+  Above:
+
+  - tmuxp loads a file named _apple.yaml_ from a current directory
+  - tmuxp built a tmux session called _banana_.
+  - `-d` means _detached_, loading in background
+
+  ```console
+  $ tmux attach -t banana
+  ```
+
+  Above: Use `tmux` directly to attach _banana_.