Add two files and one folder from doc building to git ignoring (#482)

Author: gepcel

.gitignore

@@ -22,6 +22,9 @@ docs/_build
 docs/_static/ultraplotrc
 docs/_static/rctable.rst
 docs/_static/*
+docs/gallery/
+docs/sg_execution_times.rst
+docs/whats_new.rst
 
 # Development subfolders
 dev