add substitutions

Signed-off-by: Takagi, Isamu <isamu.takagi@tier4.jp>

Author: isamu-takagi

system/diagnostic_graph_aggregator/doc/format/path.md

@@ -8,3 +8,12 @@ The structure of the subgraph file should be [graph object](./graph.md).
 | Name   | Type     | Required | Description                    |
 | ------ | -------- | -------- | ------------------------------ |
 | `path` | `string` | yes      | The file path of the subgraph. |
+
+## Substitutions
+
+File paths can contain substitutions like ROS 2 launch. The supported substitutions are as follows.
+
+| Substitution                  | Description                      |
+| ----------------------------- | -------------------------------- |
+| `$(dirname)`                  | The path of this file directory. |
+| `$(find-pkg-share <package>)` | The path of the package.         |