Improve gitignore file comments and organization.

* Add a comment describing what the files does.
* Add sections to organize patterns.

.gitignore

@@ -1,3 +1,8 @@
+# This file specifies intentionally untracked files that Git should ignore.
+# Files already tracked by Git are not affected.
+# See: https://git-scm.com/docs/gitignore
+
+## Python ##
 __pycache__
 .mypy_cache
 .python-version