gitignore: Give some organization to the root .gitignore file.

Also add a comment explaining an important non-obvious wrinkle in how
paths in the format are interpreted, and adjust some paths to a
consistent style.

.gitignore

@@ -1,36 +1,59 @@
+# Quick format and style primer:
+#
+#  * If a pattern is meant only for a specific location, it should have a
+#    leading slash, like `/staticfiles.json`.
+#    * In principle any non-trailing slash (like `zproject/dev-secrets.conf`)
+#      will do, but this makes a confusing pattern.  Adding a leading slash
+#      is clearer.
+#
+#  * Patterns like `.vscode/` without slashes, or with only a trailing slash,
+#    match in any subdirectory.
+#
+#  * Subdirectories with several internal things to ignore get their own
+#    `.gitignore` files.
+#
+# See `git help ignore` for details on the format.
+
+## Config files for the dev environment
+/zproject/dev-secrets.conf
+/tools/conf.ini
+/tools/custom_provision
+
+## Byproducts of setting up and using the dev environment
 *.pyc
-*~
-/prod-static
-*.sw[po]
-*.DS_Store
-.kdev4
-.idea
-zulip.kdev4
-coverage/
-.coverage
+
 /queue_error
+.vagrant
+/var
+
+# Static build
+*.mo
+npm-debug.log
+/node_modules
+/prod-static
+/staticfiles.json
+/webpack-stats-production.json
+
+# Test / analysis tools
+.coverage
+coverage/
+
+api/bots/john/assets/var/database.db
+
+## Files left by various editors and local environments
+# (Ideally these should be in everyone's respective personal gitignore files.)
+*~
+*.sw[po]
+.idea
+.kdev4
+zulip.kdev4
 .kateproject.d/
 .kateproject
 *.kate-swp
 *.sublime-project
 *.sublime-workspace
-.vagrant
-/zproject/dev-secrets.conf
-static/js/bundle.js
-static/generated/emoji
-static/generated/pygments_data.js
-static/generated/github-contributors.json
-static/locale/language_options.json
-static/third/emoji-data
-static/webpack-bundles
-/node_modules
-/staticfiles.json
-npm-debug.log
-*.mo
-var/*
 .vscode/
-/webpack-stats-production.json
-tools/conf.ini
-tools/custom_provision
-api/bots/john/assets/var/database.db
-static/locale/en
+*.DS_Store
+
+## Miscellaneous
+# (Ideally this section is empty.)

static/.gitignore

@@ -1,2 +1,10 @@
 min/
 source-map/
+/js/bundle.js
+/generated/emoji
+/generated/pygments_data.js
+/generated/github-contributors.json
+/locale/language_options.json
+/third/emoji-data
+/webpack-bundles
+/locale/en