docs: add supported QGIS environment variables

Starting with QGIS_CUSTOM_CONFIG_PATH

docs/usage/settings.md

@@ -24,3 +24,13 @@ Some others parameters can be set using environment variables.
 | `QDT_LOCAL_WORK_DIR` | Local folder where QDT download remote resources (profiles, plugins, etc.) | `~/.cache/qgis-deployment-toolbelt/default/` |
 | `QDT_LOGS_DIR` | Folder where QDT writes the log files, which are automatically rotated. | `~/.cache/qgis-deployment-toolbelt/logs/` |
 | `QDT_QGIS_EXE_PATH` | Path to the QGIS executable to use. Used in shortcuts. | `/usr/bin/qgis` on Linux and MacOS, `%PROGRAMFILES%/QGIS 3.28/bin/qgis-ltr-bin.exe` on Windows. |
+
+----
+
+## QGIS environment variables
+
+Some of the QGIS environment variable applies to QDT:
+
+| Variable name       | Description            |
+| :------------------ | :----------------------: |
+| `QGIS_CUSTOM_CONFIG_PATH` | Used to customize the path to the folder where QGIS stores the user's profiles. See [upstream documentation](https://docs.qgis.org/3.34/en/docs/user_manual/introduction/qgis_configuration.html#profiles-path). |