Remove `.DS_Store` files and update `.gitignore` (#8971)

The `.DS_Store` files were accidentally added in #8966

.gitignore

@@ -12,6 +12,11 @@
 
 /docs/_site/
 /docs/.sass-cache/
+/docs/.bundle/
+/docs/.jekyll-metadata
+
+# OS files
+.DS_Store
 
 # Temp files
 *~