I18n/locales exploration

[eelcoj]

This PR is to explore how to add i18n/locale to Perron. Since Perron is just Rails most of the i18n code can be used.

But how to structure the content files? Currently leaning towards app/content/:collection_name/:locale/*/*.* (another option would be app/content/:collection_name/*/*.:locale.md). The former logic is now added in this PR and could be shipped as-is.

Tentative implementation then looks like:

# config/routes.rb
scope "(:locale)", locale: /de/, defaults: { locale: :en } do
  resources :articles, path: "docs", module: :content, only: %w[index show]
end

# config/initializer/locale.rb
I18n.available_locales = [:en, :de]
I18n.default_locale = :en

Biggest unknown now is how to add these locale-based routes to the build step.

Closes: #128