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/application.rb

@@ -1,4 +1,10 @@
+# This file runs after config/boot.rb and before config/environment.rb. It loads Rails, loads the gems, loads the
+# Danbooru configuration, and does some basic Rails configuration.
+#
+# @see https://guides.rubyonrails.org/initialization.html
+
 require_relative 'boot'
+
 require "rails"
 require "active_record/railtie"
 # require "active_storage/engine"
@@ -12,8 +18,11 @@ require "active_job/railtie"
 require "rails/test_unit/railtie"
 # require "sprockets/railtie"
 
+# Load the gems for the current Rails environment from the Gemfile.
 Bundler.require(*Rails.groups)
 
+# Load the default Danbooru configuration from config/danbooru_default_config.rb and the custom config from
+# config/danbooru_local_config.rb.
 begin
   require_relative "danbooru_default_config"
   require_relative ENV.fetch("DANBOORU_CONFIG_FILE", "danbooru_local_config")