`rails_guides/helpers.rb` requires `YAML` to run.

Resolve the following error when running `bundle exec rake guides:generate:html`:

```
/Users/senny/.rbenv/versions/2.0.0-p353/bin/ruby rails_guides.rb
Generating 2_2_release_notes.md as 2_2_release_notes.html
/Users/senny/Projects/rails/guides/rails_guides/helpers.rb:17:in `documents_by_section': uninitialized constant RailsGuides::Helpers::YAML (ActionView::Template::Error)
	from /Users/senny/Projects/rails/guides/rails_guides/helpers.rb:32:in `docs_for_menu'
	from /Users/senny/Projects/rails/guides/source/layout.html.erb:56:in `block in ___sers_senny__rojects_rails_guides_source_layout_html_erb___3094858039481335962_70118647133480'
	from /Users/senny/Projects/rails/guides/source/layout.html.erb:54:in `each'
	from /Users/senny/Projects/rails/guides/source/layout.html.erb:54:in `___sers_senny__rojects_rails_guides_source_layout_html_erb___3094858039481335962_70118647133480'
	from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:143:in `block in render'
	from /Users/senny/Projects/rails/activesupport/lib/active_support/notifications.rb:161:in `instrument'
	from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:337:in `instrument'
	from /Users/senny/Projects/rails/actionview/lib/action_view/template.rb:141:in `render'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:61:in `render_with_layout'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:47:in `render_template'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/template_renderer.rb:17:in `render'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/renderer.rb:42:in `render_template'
	from /Users/senny/Projects/rails/actionview/lib/action_view/renderer/renderer.rb:23:in `render'
	from /Users/senny/Projects/rails/actionview/lib/action_view/helpers/rendering_helper.rb:24:in `render'
	from /Users/senny/Projects/rails/guides/rails_guides/markdown.rb:160:in `render_page'
	from /Users/senny/Projects/rails/guides/rails_guides/markdown.rb:25:in `render'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:205:in `block in generate_guide'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:195:in `open'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:195:in `generate_guide'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:144:in `block in generate_guides'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:142:in `each'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:142:in `generate_guides'
	from /Users/senny/Projects/rails/guides/rails_guides/generator.rb:97:in `generate'
	from rails_guides.rb:63:in `<main>'
rake aborted!
Command failed with status (1): [/Users/senny/.rbenv/versions/2.0.0-p353/bi...]
/Users/senny/Projects/rails/guides/Rakefile:11:in `block (3 levels) in <top (required)>'
Tasks: TOP => guides:generate:html
(See full trace by running task with --trace)
```
docs, reference to ruby-lang.org. refs #13492. [ci skip]
docs, revisit polymorphic associations with STI example. [ci skip]

This is a follow up to #13926.

/cc @fxn