static: Clean up organization of .gitignore.

static/.gitignore

@@ -3,14 +3,21 @@
 /source-map/
 /webpack-bundles
 
-# Other
+# Generated static files
+
+# Copied zulip_bots package
+/generated/bots/
+# From emoji
 /generated/emoji
+# From passing pygments data to the frontend
 /generated/pygments_data.js
+# From `tools/update-authors-json`
 /generated/github-contributors.json
+
+# Generated i18n data
 /locale/en
 /locale/language_options.json
 /locale/language_name_map.json
-/third/emoji-data
 
-# Static files from the zulip_bots package
-/generated/bots/
+# Legacy emoji data directory
+/third/emoji-data