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/README.md

@@ -0,0 +1,16 @@
+# Webpack
+
+This directory contains configuration for Webpack. Webpack is used to bundle Javascript and CSS
+assets in production. In Rails, Webpack is wrapped by Webpacker, a layer that tries to provide
+sensible defaults for Webpack.
+
+# See also
+
+* [app/javascript](../../app/javascript)
+* [config/webpacker.yml](../webpacker.yml)
+
+# External links
+
+* https://guides.rubyonrails.org/webpacker.html
+* https://github.com/rails/webpacker#webpack-configuration
+* https://webpack.js.org/configuration/