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/initializers/README.md

@@ -0,0 +1,15 @@
+# Initializers
+
+This directory contains initializers run by Rails during bootup. Initializers are generally used to
+configure libraries or other global settings during application boot.
+
+# See also
+
+* [config/boot.rb](../boot.rb)
+* [config/application.rb](../application.rb)
+* [config/environment.rb](../environment.rb)
+
+# External links
+
+* https://guides.rubyonrails.org/configuring.html#using-initializer-files
+* https://guides.rubyonrails.org/initialization.html