Launch file documentation for jupyterlab-python (PrairieLearn#11162)

* Launch file instructions

* Small formatting tweaks

* Update workspaces/jupyterlab-python/README.md

Co-authored-by: Nathan Sarang-Walters <nwalters512@gmail.com>

---------

Co-authored-by: Nathan Sarang-Walters <nwalters512@gmail.com>

workspaces/jupyterlab-python/README.md

@@ -0,0 +1,16 @@
+# workspace-jupyterlab-python
+
+To automatically open a file when the workspace is started, set the `LAUNCH_FILE_NAME` environment variable in your question's `info.json`:
+
+```json
+{
+  "workspaceOptions": {
+    "image": "prairielearn/workspace-jupyterlab-python",
+    "environment": {
+      "LAUNCH_FILE_NAME": "some_file.ext"
+    }
+  }
+}
+```
+
+Replace `some_file.ext` with your file. It is expected that this file exists inside of the `/workspace` directory of the question. See the [workspaces documentation](https://prairielearn.readthedocs.io/en/latest/workspaces/) for more details on environment variable usage.