docs: document config/ and some directories in app/.

* Add README files to several directories in app/ giving a brief
  overview of some parts of Danbooru's architecture.
* Add documentation for files in config/.

config/webpack/development.js

@@ -1,3 +1,5 @@
+// @see https://github.com/webpack-contrib/eslint-webpack-plugin
+
 process.env.NODE_ENV = process.env.NODE_ENV || 'development'
 
 const { merge } = require('@rails/webpacker')