add __marimo__/cache/ to Python .gitignore

The marimo notebook maintains a cache of binary data in
__marimo__/cache/, and it recommends users to add this directory to
their .gitignore.

Documentation: https://docs.marimo.io/api/caching.html#caching-variables-to-disk

Python.gitignore

@@ -78,6 +78,10 @@ target/
 # Jupyter Notebook
 .ipynb_checkpoints
 
+# marimo notebook
+#   marimo maintains a cache of binary data in __marimo__/cache/
+__marimo__/cache/
+
 # IPython
 profile_default/
 ipython_config.py